I shoot with both the 85/1.5 and the 100/2, and in my experience (and opinion):
-- At wide apertures, the 100/2 is somewhat sharper in the center and considerably sharper at the corners. The 85 is okay, but the 100 is noticeably better.
-- At moderate to small apertures, the differences level out and both lenses perform very well.
-- Neither of them has quite as much contrast as a modern lens in the same category (my main basis for comparison being the 85/1.4 Minolta I used to own and the 85/1.8 Nikkor I have now) but the 85 is acceptable and the 100 is good. (If you're nuts for contrast and texture and want a vintage lens, you're probably better off scaring up a 105/2.5 Nikkor -- but the Canon is 1/2 stop faster and is still pretty good for contrast.)
-- The 100 is much better at full aperture than the 90mm f/2 Summicron I used to own (first generation model with removable lens head.) The Summicron produces a halo around bright highlights, while the Canon's image is clean.
Overall, I'd say that of all the vintage teles I've used, the 100/2 comes closest to the sharpness and contrast of a modern optic, while still retaining something of a "vintage" look. With most older high-speed medium teles, you find yourself saying "It performs really well for a lens designed in 19__," while with the Canon you can say "It performs well," with no qualifications.
Another advantage vs. the 85 (since you mentioned that you're a P owner) is that you don't need an accessory viewfinder for the 100, but you do for the 85!
One downside (also true of the 85 and most other Canon RF tele lenses): it's got a single-helical focusing mount, so the aperture ring rotates as you focus, and you have to hold the focus ring still with your free hand to change apertures! Lots of other oldie lenses are like this, too, but it seems odd with the 100/2 because it is so modern in most other respects...
