I have an R2a - I think all the Rs are all built on the same chassis, but I'm sure someone can correct me if I'm wrong - and the build quality is fine; there's nothing about it that I think "I wish that was better quality". It feels strong and robust.
On the other hand I have never so much as held a Leica that I can remember, so I have nothing to compare.
What will matter the most, I suspect, is what you think when you physically pick one up - it'll either suit you or not. If you don't want to put it down again, it's probably the right camera. Same goes for the M6 or the Ikon