On FF the Takumar 135/3.5 does much better than on apsc imo.
For the same framing, the DOF is shallow enough such that f3.5, f4 still gives good isolation.
Bokeh is rather well controlled right from f3.5, f4, imo (some faster 135mm are more busy for half/full body shot distances, until stopped down a little).
The Takumar 135/2.5 (M42 mount) has 2 versions.
Both are good imo.
For portability, I take the Takumar 135/3.5 more often these days.
If its more specific portrait type shoots, I'd use the 135/2.5
There is a K-mount Takumar 135/2.5, which is said to be the weakest of the Pentax 135mm.
This is the cheapest of the lot.