Good question!
Price is the major difference.
Kidding... The metering led view through the viewfinder is different. In R2, as what Mr Gandy described: "3 Red LEDs in lower portion of viewfinder, 2 arrows on either side, with a round circle in the middle. "+" on right side, "-" on left side, correspond to shutter dial direction"
On R2M & R3M, it shows: -2 . . -1 . . 0 . . +1 . . +2
That's exterial.
The construction is different. When you hold one, you can feel the difference.
Internally, the mechanism different. With each new model, the internal mechanism is modified after feedback from users. (I only learnt this in the past 2 days after some training with the Cosina engineers). The R2M & R3M is said to be heavier and better built than the R2. Shutter from some users is said to be quieter too.
I do not own the R2, so, that's as far as I can tell you cosmetically.
R2 is on longer in production for a long time. If you find one, most probably it's 3 to 4 years old or it's old stock from the dealer. The models that are available new are: R2A,R3A (grey & matt black) R3A (grey & matt black) R2M,R3M,R4M (black paint) & R4A (matt black)
Hope that helps!