Both lenses show a good contrast and colours. I can't see much of a difference here.
Indeed, the Zeiss and Pentax brands are well-known in this respect, and the two live up the expectations here.



The first shots I took didn't say much about the bokeh, but now I'm here to amend (someone may have read a first version of this page, where this topic was not covered).
I'll not delve here into the explanation of what bokeh (quality of the out-of-focus areas) is, let's just say that there are some who value this parameter over any other characteristic in a lens. Evaluation is subjective, but in general what you want are creamy background and foreground areas, without harsh lines or nervous strikes.
There are some "odd" types of bokes, such as the "swirly" bokeh, that you may like or not, anyway these two lenses do not display any special behaviour.

Let's go, then!...
I took some shots but one can tell the whole story, it portraits daisies in a green field; here things get definitely interesting! The Pentax-M behaves perceivably better than the Zeiss, at all of the wide apertures (up to f/4).

At f/2 the Pentax runs solo, and you can see both the high quality of the blur and, in the focus plane, some strong blue haze (definitely the worst issue of this lens). Just for the bokeh, this is a very satisfatory outcome:



The relative softness and the excellent bokeh make this item a very desireable portrait lens, indeed!

Moving on f/2.8, the Zeiss comes into play but the Pentax-M is better, according to me. The first image is from the Pentax, then comes the Zeiss:





It is hard to tell the whole story from these crops, but the Zeiss shows a bit more of harshness both in the background and in the foreground, but don't think it is bad: it is a fair show, aligned with most other lenses, but the Pentax-M is better, very good (again).

...and finally, at f/4, here we go again, with the SMC Pentax doing very well, even better than expected, whereas the Zeiss is just average or unpleasant in some spots (see "back"):





There is no doubt, the Pentax-M 85mm f/2 is a winner here!

This is the studio image, the shot taken with the Pentax-M at f/2:



The colours are well-balanced, the background is pleasant (but the distance from the subject is not big), the contrst is OK.
The detail that follows is taken at full aperture (f/2), at 100% crop, and you can see by yourself that it is very good indeed!



We're going on with the Pentax... what is you see is what came out of the camera. The following crop, again at 100%, comes with a bit more contrast and some sharpening applied; given the aperture, again... not bad!



Near the border of the short side of the image, the outcome is surprisingly good, too, just veiled by some minor haze; there is some light fall-off:



...but there is a dark side, take a look at the ring of the can lid, that is in the foreground:



The chromatic aberrations are evident, and yet, in most cases, they won't disturb the general view. It's up to you to decide how relevant this aspect is. Take a look at this other part:



...yes, there is a bit of blue haze also in here, not far from the centre, but really it is "nothing".

Jumping to f/2.8, sharpness improves and aberrations in the centre are gone. The resolution, indeed, is very good:



And it's excellent also near the border of the short side of the image, as you can see here (I have applied some extra sharpening):



Alas, the same pictures I couldn't take with the Zeiss because the light went down abruptly, but a few days later I tried again, with a slightly different setup... not much different, indeed:




The Pentax runs alone at f/2, its full aperture:

and confirms the good performance at full-aperture. The violet halo is evident in the corner; expect it to get worse on full-frame.


At f/2.8, we can put the competitors one against the other:

Pentax-M 85/2 at f/2.8	Zeiss Sonnar 85/2.8 at f/2.8

Watching the image in general, the Sonnar produces a crisper impression, must be the famous "3D pop up" the Zeiss is famous for! Or it is simply the better sharpness, as the images easily prove to be there.
The Zeiss also is fringe-free, while the chromatic aberrations still affect the Pentax... there is no contest here!
On its side, the Pentax seems to deliver some more contrast, but the effect is compromised by the inferior sharpness.
In brief, the Pentax is still a good lens, and yet it must bow to the king!

f/4 is an aperture I don't use often with legacy lenses: I frequently need to open the blades some more, even to full aperture, otherwise, in plain light, I'd go for f/5.6 or f/8 that is where most lenses show their best sharpness.
My setup failed again, all images but one (taken with the Zeiss Sonnar) display camera shaking, so I will explore the f/4 case with a third attempt and a tripod, or better light! Let me just show you the crop from the only good image and how the Sonnar is flawless:

crop (Zeiss Sonnar 85/2.8 at f/4)	comment
	not much to say about the center, where the streaks of the pen ink and of the pencil are well evident
	the middle of the short side. Here, and also in the corner, there isn't the slightest sign of colour aberrations
	corner crop: see what I mean? The lens is probably outclassing the resolving power of the sensor, the details of the separation between the paper of the book and the cloth are impressive. On APS-C, there is no difference between the centre and the corner!
	to further underline the perfection of the image, take a look at the tiny hair in this crop, that is almost from the corner. In the picture, this grey-on-white thin filament is just one or two pixel wide, and yet you can see it... this what the Zeiss can do!

We also want to take a look at what happens when focusing at infinity. Here the situation is less simple than it seems!

At f/2.8, there is a real head-to-head. Though the Zeiss has the upper hand, again, in terms of aberration control, the Pentax seems a tad better in terms of sharpness and contrast. From the purely optical point of view, I'd still prefer the Zeiss, since I hate to fix aberrations, but at least with my gear I'd rather take the Pentax with me, because it's lighter and because focusing to infinity is immediate (see the Zeiss quirks with my gear).
Is f/2.8 useful with landscapes? Not usually, but you never know, you may want to separate the planes by shrinking the focus range.
One specific case is night landscape. In such case I'd still bet on the Zeiss, because the Pentax aberrations would be unpleasant with light points.

Pentax-M 85/2 at f/2.8

the centre is not stellar but very good indeed amd strong lights don't leak into darker areas. Contrast and colours are OK

the middle of the short side is very good, too. Look at how the tiles of the red floor are divided by slightly darker, thin lines: the contrast and sharpness that you need are all there

even the corner, though on APS-C, is very convincing. Yes, there is an unpleasant blue haze but post-processing can easily get rid of it, I tried myself with Rawtherapee

Zeiss Sonnar 85/2.8 at f/2.8

the centre is more or less on par with that of the Pentax, possibly a bit better (look at the triangular antenna that points to the right), but at times it seems the opposite. I really cannot tell

the short side, again, shows more or less the sharpness of the Pentax. You can see the lines that separate the tiles of the red floor: the contrast is excellent. I'd say it's less sharp, but... read about corners

here is the point with this image: I took two samples (I ususally take two, each time refocusing), and where I had the best centre, I couldn't gain the best corner, and viceversa. This is the worst corner of the two, but also is the one with the best centre; it also shows some coloured halos

this is the corner from the second image. It is better, impressive I'd say, and yet the centre from this one I have discarded. So, the Sonnar at infinity seems to suffer from a bit of sphericity, just a bit, where the Pentax is more consistent

Going on, what happens at f/4 is very interesting!

Pentax-M 85/2 at f/2.8

as for sharpness, the centre is excellent, also the contrast reaches ideal values

the tiles of the red floor in the middle of the short side are even better separated than at f/2.8

the corner sharpness is impressive, so is the contrast, and also the blue/magenta bleeds are almost gone

Zeiss Sonnar 85/2.8 at f/4

the sharpness displays marginal improvement at f/4 for the Zeiss, I think we're almost outresolving the sensor capacity. And yet, the Pentax has been more convincing here

the middle of the short side is again excellent, but I prefer the Pentax, again, by a small margin

the corner sharpness is impressive. According to where you look, the Pentax or the Sonnar may win, but generally I prefer the Sonnar. Notice that the image is slightly darker, indeed in this case the Fuji-X opted for 1/1100 sec for the Sonnar and 1/950 for the Pentax

Long story short (for the moment I omit the images), at f/5.6 and f/8 the two lenses become boringly almost perfect, only in the corner f/8 is doing better, but it's really by a hair.
It is really hard to say which one I'd pick at these apertures. Not all of the details are rendered the same, though.
Though the red floor of the short side is constantly better for the Pentax (maybe the contrast for the red nuances is better?), the center of the Sonnar is unmathed for a couple of details.
At f/11 both get slightly worse due to diffraction.

Let me remind you that these tests have been performed with an APS-C camera, namely the Fujifilm XT-20 with a 24MP sensor.
For full-frame output, you have to look somewhere else, sorry!

I am not indulging in praises for the Carl Zeiss Sonnar 85mm f/2.8, the purpose is rather to tell about the Pentax contender, but undoubtely the Sonnar has better characteristics from most points of view, from general image quality, especially in terms of [micro-]contrast (for which most Zeiss lenses are universally praised), not to mention control of chromatic aberrations, etc.
It's more a matter of taste, but possibly for the reliable, sturdy mechanics combined with unrivalled smoothness also the construction of the Zeiss is, again, better.

So, would you trash the Pentax-M 85mm f/2? Not at all!
First of all, the Pentax has a sublime (yes, "sublime") bokeh. It may not be on par with some "bokeh monsters" or the expensive modern primes such as the new Zeiss and the Sigma art, but it's really really good in this compartment!
On the contrary, the Zeiss is just average and missing the f/2 aperture, it cannot even compete on the widest aperture, where the out-of-focus impression is at its best.

Secondly, the Zeiss has some issues matching my gear, as explained in the beginning, and it has happened more than once that I missed the proper focus point. I can't blame it for not hitting infinity at the end of the run, because probably it is due to the length of the adapter, nor for the fall of brightness in the viewfinder, but indeed these are issues.
Also, on small cameras the weight of the Zeiss can make a small but perceivable difference.
So, yes, I may take the Pentax with me sometimes.

Moreover, the images tell that you can achieve excellent results with the Pentax product and at times it can do better than the Zeiss: though the result at a short distance crown the Zeiss, at infinity I can't says which one is the winner, and the Pentax reaches a higher level at f/4.

For shots at night, portraits, whenever lights are low or you want the shallowest depth of field, the f/2 aperture comes handy, and the sharpness of the Pentax is not shabby even at full aperture, in the centre at short distance is rather astonishing (for being f/2, I mean).
So, if you need a mid-tele that is good at f/2, this is your lens!
Consider that brighter 85mm lenses, e.g. sporting f/1.8, are very (or terribly) expensive or, in the case of some recent products such as the Samyang, are reportedly good and not expensive but a kind of hit-and-miss, likely due to scarce or null quality control. The same are usually much larger and heavier than the Pentax-M.

Finally, if you own a (Ricoh) Pentax digital reflex camera, you don't need an adapter and you can happily exploit the compactness of this 85mm. I must say it's a pity that no corrensponding Pentax-A lens has been produced, unless you fancy the Pentax-A* 85mm/1.4, that according to reviews plays in another league, costs four times and is not compact.
Anyway, the Pentax brand deserves the highest praise for still keeping the original PK mount!
So... well, in spite of the legendary T* coating of the Zeiss, its larger front lens and a price that, at the time, was far superior, the Pentax defends itself and doesn't take a hard beating.
Probably its major weakness is the pronounced colour aberration that some may not forgive, followed but softness in the corners at short distance shots; there is not much to complain otherwise: it definitely is a worthy lens, a must-keep for me.