Just like the McLaren, this comes down to the driver's ability to tame the beast.
The M3 GTR is the "every man's car," the RGT is the car for specialist who can deal with the rear-engine lay out.
Overall, the RGT should be, and probably is faster when in the right hands.
However, the average racer does not have "the right hands," and so, most of us will turn better lap times with the more stable and more predictable M3.
My pick,
The RGT
I really like the M3 GTR.
However, this comes down to looks for me.
Both cars are fast as hell and I can garuntee they will run nearly identical lap times under the right conditions so it comes down to which car looks better.
For me the better looking car is the RGT.
Now compare the M3 CSL and the RGT and we will really have a stale-mate.