i have had both these engines, and i definatly vote RB!
the GRP looks like a gem, but for me it stopped there.....
it was difficult to break in, it would rev iraticly, and i always was chasing the tune........it did have a smooth power band but wasnt overly poweful......iv seen more than one of them get smoked down the straight like it wasnt even fair by an x-tech.......
the RB is just the opposite........not much bling, but is verry consistant, easy to tune once you get it right, but the S7 isnt a powerhouse either and is smooth as well......