Machine, everybody is going to disagree on this one, but I've found the Gnarly Rev (desert) to have unlimited overrev, but that may also be from the porting on my bike. But I can tell you that moving from a stock exhaust to a gnarley desert, it increased the overrev by about 50%. Did not notice a diffrence in power anywhere else.
I've also been told that the Gnarley Rev is just a thick walled fatty with a little more overrev.