According to Erwin Puts late generation R lenses are very good. I would suggest that R-lenses would be preferable on NEX because lenses designed for SLRs generally work better on the NEX. The microlenses on NEX are not optimized for M lenses but would work well with R lenses.
I'm sure someone has run real-world shooting tests on equivalent Leica M and R lenses; I'm looking to add some Leica glass for my Sony Nex, and there are adapters for both lenses. I am drawn to the mechanical simplicity of the M lenses, but here in Russia, an R lens is significantly cheaper than its M counterpart. Any thoughts?