I have experience on both, and they're both slow as hell. I do feel like the xr has better aftermarket support since its had a stronghold in racing since the dawn of time, and the grom is a newer model. I believe the grom is quite a bit heavier, which would offset the slight hp advantage. I think since the xr's chassis is longer, it's a bit more comfortable to work with. Which is an advantage in the xr's favor in my mind. Not to say both aren't a blast. I have a grom because its a street legal pit bike, I mean... What else do I need to say? lol. I just rode to work 20 some miles on it yesterday and laughed the whole time.