Id go with the 5 horse, but,
I think a small header, no governor, new aircleaner and a few other things might help the little 3 pony breathe better and therefore produce a little more power, the 5 horse would be vastly cheaper if you want EXTREME performance, because custom machining for rods and cams will cost you a nice pretty penny,
try searching "3 horse mods" here or on 4cylcle.com There are people that have done it, and a few that have produced some very impressive dyno results