Got both.
For me the RB C6 has more, both TOP and BOTTOM end. The GRP is much easier to drive with a much better throttle curve, the C6 is pretty aggressive. I get slightly better fuel milage out of the GRP.
If I could have only one it'd be the RB, I feel it's slighly better made as I've had one go 5gallon race tuned, I've also had a GRP go bad with 2gallon on it, BUT they warrantied it, so ain't all bad. I would still buy another
Both are VERY good motors, If your not a seasoned racer I'd get the GRP I believe you'll make it around the track faster with it, If your a VERY controlled driver the C6 maybe the way to go...
Rod