Truth is, the smaller engine is better tuned and produces more power. The bigger engine produces more torque, so it may have a small advantage from a standing start, but the greater HP that the 790CC engine has will make itself know by the time you shift to second. Also, because of the more aggressive valve timing, the 790 responds better to modification.
If you like the wheels and feel of the America, by all means get it. The America rear wheel and Thunderbird Sport front wheel will fit the Speedmaster with little problem and the America handlebars will bolt on with a little rerouting and extending of wiring and cables. The gotcha is that it will cost you. Because you will be able to use it for other things as well, it would probably be a better bargain to get an America and a home powdercoating system and black out the engine yourself.
