Here are the differences that matter.
1. Seating position, rider: The America has a more upright position and rather wide handlebars. The Speedmaster leans you forward a bit and the bars are noticably narrower.
2. Seating, Pasenger: The Speedmaster has a 1 piece "gunfighter" style seat. Many passengers find those uncomfortable and feel like they might fall off. The America has a 2 piece seat with a flatter, wider rear portion. Some like it, some would prefere the bigger, softer, (uglier to some) optional touring version. Also, the rear seat on the America can be removed leaving the solo front seat. Many like the look of that.
3. Power/Performance: The Speedmaster has more torque and is geared lower, giving it the advantage from a standing start. The America has more peak power (due mostly to differences in valve timing) and has the advantage at higher speeds.
4. Wheels: The Speedmaster has cast alloy wheels. This permits the use of tubeless tires. For the most part, these wheels are not repairable and must be replaced if they receive moderate damage. The America has wire spoke wheels. These require inner tubes. Some damage can be corrected by retuning the spokes. Because of the difference in wheel weight, the America will ride slightly smoother.
5. Brakes: The Speedmaster has one more brake disk. If you tend to push the bike hard a lot of the time, this can be an advantage. If you ride at more "normal" speeds, having to replace one less pair of brake pads gives the America a little advantage. For the average rider, the single disk on the America is more than enough.
Chrome VS Black anodized trim. All depends on what you like to see and which flavour polish you like to use.
So, pick which of the above tradoffs appeal to you. By all means, test ride both because the seating position is the difference that will be most important in the long run. If you like the seating position of one, but just have to have the look of the other, buy the look and then change the handlebars and seat. You might be able to get the dealer to swap them out and give you an allowance for the new parts that were taken off, but don't bet on it.
