HI,
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..
Hope this helps you.
Andrew