The 35/2 concave front lens is very sharp. I use it with color print film and any color cast is easily filtered out. I wouldn't use it with slide film. Slide film is very expensive now and other lenses are more suitable. After the concave front lens there was a convex front 35/2 FD SSC, which is also very good. The later 35/2 New FD is smaller and lighter and also sharp but not as mechanically sturdy as the earlier two lenses.
Between 1975 and 1977, Pentax made a 35/2 SMC Pentax K mount lens. It looks like a carry-over design from the last M42 35/2 SMCT. There were then two versions of the smaller 35/2 SMC Pentax-M. I have one of those and it is very good. Other choices, if you don't need as much speed are the 35/2.8 SMC Pentax-M and the older 35/3.5 SMC Pentax. Both are good but the f/3.5 model is exceptionally good. It traces its design back to the 1950s. I have several M42 versions, which are also excellent.
For the Canon, if you do not intend to use slide film, the concave front lens (FD SSC) would be fine. If you will use slide film, I would recommend the convex front FD SSC. For the Pentax, I would get any 35/2 SMC Pentax-M that is in good condition.