Main differences are screw vs M mount and aperture priority auto exposure. Some people say that he R3a is also built better. I have an R and it is no all that bad. You can use pretty much every screw mount lens on a M mount camera with an adapter, but you can't go the other way. The Rs can be had for much less money. but are only available used.
In addition to what the others said, the cameras seemed to use more metal and less plastic in successive models.
I think an R2 or one of the other M-mount cameras would be the better choice, if only because you have the option of using either M-mount or screw-mount lenses.