The biggest difference is how you sit. Ride both and pick the one that is most comfortable. The next biggest difference is that Speedmasters are geared lower, giving them a bit better acceleration and less top speed. Also the look is different, but that doesn't matter much if you pick the one you like to ride because you can't see much of the bike from the seat. Oh yeah, the Speedmaster comes with a tach and it is an option on the America.
