There is no difference. All Leica-made lenses are either -M or -R (and probably -S coming up). I believe the 4/3 lenses are marked "D".
There are two interesting 50 Summiluxes:
- Leica Summilux-M 50mm f/1.4 (pre-Asph)
- Leica Summilux-M 50mm f/1.4 ASPH
The latter is perhaps the best normal lens ever made, an astoundingly good lens with awesome boke and sharpness. However, wide open, some prefer the also very good pre-ASPH. It has a little of that old glow. The latest version with the pull-out hood focuses closer and is quite sought-after.
If you have seen higher prices from lenses marked -M, it is likely coincidence (since they all are, except the very first versions from '59-'61, order number SOOME, 11114/11113, SOWGE, 11014, these can be recognized by their scalloped focusing rings), or it just reflects that people selling more expensive copies are more careful with their writing
All 50 Luxes from '61 and up to the latest pre-ASPH (11114, 11868, 11856, 11869, 11621) have the same optical formula. The early Lux is not nearly as good as the later two.
You will notice that the order number 11114 appears in both lists. Leica made a gliding switch and didn't confirm until '68 that this had happened, so I would recommend avoiding this, unless you have access to an expert who can tell the difference via serial numbers or possibly other means.