It all depends on what you are going to do with this machine. I rode an 800 and I own a 650 HO and I'll tell ya, I saw very little difference when riding in trails.

If you intend to work with it hard, then go for the 800 , there is a few horsepower extras on this one than the 650.

Personally, 650 HO EFI is the perfect engine for me for trails anyway.