Rack mount- generally perceived to have better engines than the center mount "Happy Time" engines. Available in chain drive, Belt drive, Friction Drive (Friction drives eat tires faster than the other 2 methods). Bike not as well
balanced using a rack mount as opposed to a center mount (engine is higher up and in the rear)....Also, harder to get on the bike because you have to swing your leg over the rack mount. Limits cargo capacity because you lose the rear rack area to the engine.
Frame mount: better balance, Looks nicer in my opinion. space to carry stuff on a rear rack if needed....generally kits are cheaper..
I have both. A China engine in the frame and a Titan Rack mount.
I ride the Titan a lot more because it's a whole lot smoother but I like my china 2 stroke too because it has it's own character.
For the bike trails the Titan wins hands down because it's quieter and it's actually more stealth when your coming towards a roadie. They can't see my engine behind me.
My Titan also makes my bike look like an engine assist bicycle and less like a wanna be motorcycle.