In my experience, the M versions has been more frequently redesigned, which has given the M lenses an edge over the (mostly older) R designs.
The M 35mm f1.4 versions are always better than the R version.
The M 50mm f1.4 and f2 are better than the R versions.
When they are both from the same generation, there is little difference.
The R 80mm f1.4 is equal to the 75mm f1.4. The latest 28mm 2.8 are both very similar. The Apo 90mm f2s are very similar.
The real R heros in my view are the R 19mm, the R 60mm Macro, the 180mm APO 2.8 and the Modular APO-Telyts.