I don't know anything about the Porst, other than what I read here:
https://www.pentaxforums.com/forums/pentax-beginners-corner-q/108320-regardin...nual-lens.html
You may find this helpful.
Regarding the M135/3.5, I have one of these, amongst several other 135s, and really like the fact that it's
much smaller and lighter than the others. This makes it a popular choice for putting on my camera!
I'm happy with the IQ of this lens (I haven't any images which have "awkward" bokeh situations), but would expect my S-M-C Takumar, Pentax K and Carl Zeiss Jena versions of 135/3.5 to outperform it in a controlled comparative test (which I haven't yet got round to doing).
Just to summarise the salient features of the 135s I mentioned:
M135/3.5: very compact and lightweight
K135/3.5: excellent IQ, slightly heavier to focus
S-M-C Tak: excellent IQ, wonderful smooth focusing, lovely build quality, but M42
CZJ: excellent IQ, all controls heavier, poorer build quality, M42 of course
If you're after nice soft bokeh, I'm thinking that the CZJ might be your best (but most expensive) option:
Bokeh Test
Do note that it can be difficult to provoke "bad bokeh" in real-life situations, and that a lens which is known for "bad bokeh" may actually perform satisfactorily most of the time. And bokeh is a subjective thing, anyway.