OK, here's my take on it - based on the fact that I originally bought an R, then updated to an R2.
The differences have been described in many places, but they can be summed us as R: Screw Mount, 'Plastic' top and bottom and rear plates, single stroke advance. R2: Bayonet Mount, Metal top and bottom and rear plates, ratcheted advance.
OK, so far hoopy. What that doesn't tell you is the difference in feel between the two. The R2 is heavier, solider, and has a much better feel - whatever that is!
That's not to say there's anything at all wrong with the R. I used my R with CV 35/2.5 and CV 75/2,5 lenses and got some great shots. The camera felt good in my hands, and handled very nicely indeed.
Until.........
I needed a faster lens. And when I went into the local Voigtlander dealer, they only had the Ultron 35/1.7 mounted on an R2. Bad bad mistake. It really does feel THAT much better.
Does it take better pictures? Not really......the shutter, viewfinder, rangefinder is identical. Some people have claimed that the R2 is quieter, or has a better shutter release - personally I think that's manufacturing tolerance; I've handled examples that both prove and disprove the point.
Is the metal vs plastic important? Dunno - Plastics now days are pretty damn tough and long lasting.
If feel is important to you, handle both and see.
If you are going to change lenses often, the Bayonet mount may be vital - but then you have to add the cost of an adaptor to each lens. Of course, if you want a Bayonet mount body for your M-mount Summicron 35 it's gotta be the R2.
One other thing is appearance - the R2 is not available in Silver/Chrome - so if you like that look you either go for an R or a Rollei RF 35 (Bessa R2 in drag) at an expensive price.
The final arbitrer might be availability - here at least R2's are sold out, whereas you can still get R's.
I have to say tho, the current clear out price on the chrome R with the chrome 35/2.5 lens is mighty mighty tempting....
Hope this hasn't jsut confused you more!
cheers...
tim