Personal experience and opinion here - take it with a grain of salt.
I really like the 35 Ultron. Very sharp and I thought it was the perfect size lens to balance out the R2. Nothing not to like about it.
The problem with the R3, though, is that the framelines are really up against the edges of the viewfinder. With a 35, there will be parts of the frame that you will not see - to me, that's a big enough problem that I won't use a 35 with it.