The M series is the middle of the road line. They have flex fuel and EFI options, MOD decks and a couple other things.
The R series is the top of the line and you have a lot wide range of engines sizes, flex forks, suspension seats and a couple other things on top of what the two lower series have.
Go here to view a PDF of the salees brochure. Click on each picture then wheel in for a larger view of the stats.