The new Speedmasters have a bit bigger engine that produces more torque. BUT, the America produces more power because the Speedmaster has been seriously detuned to meet emissions requirements. The end result is that the SM is a bit quicker 0 - 60, but the America is faster from there up into the triple digits.
The 2 biggest differences are looks and seating position, so ride both and see which one feels like something you could ride for 3 or 4 hours at a stretch.
If you like the looks of one but love the seating position of the other, buy the looks because it isn't terribly expensive to swap the handlebars and seat.
Also, while all the available colours are quite pretty, there is a reason they call it
JET black!
