Just to make things simpler for the OP (or more complicated).
Here is the Super Takumar or SMC Takumar 85mm f1.9 optical diagram:
and here is the SMC Pentax-M 85mm f2.0 diagram:
Not exactly, but nearly the same. I figure that Pentax when they made the M serie recycled the 85/1.9 design, moded it a bit to make it even more compact, et voilá, the M85 was born.
The f1.9 or f2 hardly matters.
So the questions to ask yourself are:
Will you ever use it on a m42 body? Then go the Takumar way.
Do you want to shoot in manual with the green button? Then go the M85 way.
Do you prefere to shoot in apperture priority wide open (or just stopped down one or two steps)? Then go the Takumar way.
Is SMC important for you? Then avoid Super-Takumar version.
Is small important? Then go the M85 way?
Are you ever going to use it on any K mount film bodies? Then go the M85 way.
I think both are good lenses. In the end it may be up to what lens that first appear at the right price...