My suggestion ... based on output
Without a doubt, the lens I grab for without hesitation is the Zeiss.
Unless you are a fan of Manual Focus ... the "STF" requires a little more thought and care in the imaging process. The long and short of it is:
- The CZ 135mm f/1.8 zeroes right in and BANG, it is done. IMO, the best no-brainer experience you will have.
- Old school throws the STF 135 the ace, because you can "craft" your shot ... softening the edges of your bokeh-backgrounds and lights. Had that lens been out in the world, say back in the 70s and 80s ... it would have been the ACE of Spades and a lot of future glass probably would have been based on the very-pleasing effects it provides.
Both lenses are awesome ... and the f/1.8 also has the added edge of low light operation. The STF ... not so much. Although it states f/2.8 on the box and the lens. it really is an f/4.5, due to the design and second set of aperture blades.
Good luck with your decision. It's not a bad one to have. I couldn't decide. So, it was BOTH! :D
Did not get to the 135mm f/2.8 [4.5] STF
Well, my shot went down and because the rain was so persistent I did not really get enough distance during the shoot to make use of the SONY 135mm f/2.8 [f/4.5] STF lens.
The weirder part is that during the shoot, I abandoned the SIGMA 24-70mm f/2.8 DG EX IF HSM zoom for the fixed SONY 50mm f/1.4! I mean, the price difference here is a whole different issue. To think, I could gotten away with this with that lens is kind of ... well, sad. The amount of money we are talking about is scary. I kind of wish I had the SONY 35mm f/1.4 G for comparison reasons and it might have relaxed the distance problem, too. Obviously, with that short of a focal length, you could have a fight on your hands with the "plumping up" of the ladies, when you go in for a portrait. 50mm is kind of the minimum.
Anyway ... did the shoot, despite the rain. Got some pleasant results.
Gee, Frank ... kind of makes ya wanna shoot the 50mm, eh?