i like the that r3 is portable, i've picked it up with one hand during performances and played it with the other. the little lcd displays with the customisable knobs on the r3 are also good.
The distortions and amp modelling on the r3 are awesome, much better than the very limited overdrive on the sh-201, which sounds pretty average. the r3 just sounds better, plus you have more to work with, there are three effects layers you can run your sounds through, and there are quite a few awesome effects you can use. Where the r3 wins out is in it's sound engine. which is much better than the pc editor that comes with the r3, even though the r3 editor has a nice midi monitor that comes with it.
*also* the sh-201's vst plugin interface is awesome if you have it connected to your pc alot, you can save patches and tweaks in your software of choice. The sh-201 also works quite well performance wise, once again because most parameters are tweakable right in front of you, plus the d-beam is fun, and quite configurable. It certainly helped me understand subtractive synthesis in a simple logical way because of how it is all laid out.
I must say that as a learning instrument, the sh-201 definitely comes out on top. Sorry, I'm a bit late to the party, but I felt like adding my 2 cents.