I assume you have the so-called Leicaflex I (with the non-TTL meter and the clear-apart-from-centre-spot focussing screen), rather than a SL or a SL2.
R lenses are just as good or even better than M lenses (generally speaking).
Both lines of lenses have several "generations", each with slightly different charactersitics.
The M wide-angles have a natural advantage as far as distortion control is concerned (along with a natural disadvatange for vignetting). Not that in most cases either line is seriously affected by either.
I'm not sure about the 50mm Summicron, however the 90mm ones are the same and the 35mm ones are not.
As far as my 35mm Summicrons are concerned, I actually find my R one (type 3) much better than my M one (type 4).
R lenses suffered from poor marketing, high prices and being overshadowed by the "M myth". Quality-wise, they are absolutely first rate (with the usual model by model caveats).