The advantages you get with the SM are
1)-alloy wheels
2)-better tires (metzler)
3)-twin discs up front
4)-tachometer
The cost to upgrade an americ to these specs if it was even possible to get alloys for it would be far far more than the $600 difference (street price) between them. the question is whether these things are desirable to you. After owning a speedmaster i can say that each of those 4 things is important to me, and far more than i thought they'd be. You apparently prefer the america's looks tho, so you'll just have to weight the disadvantages and advantages. But unless you have seen them in person so far, don't count on finding the america better looking in person. Not that you necassarily will find the same thing i did, but i went in to buy an america but when i sat them side by side the SM looked much nicer to my eye. That probably would have been enough to change my mind, but then thinking about the things i listed above it became a no brainer for me.
As for the engine, it's bigger but not necassarily any better. Just different because it's got a different cam that is tuned for torque and a 16 tooth counter sprocket for even more low end grunt. I find it cruises great at 85 tho, so no compromise there. But if you like torque the SM has it. Whether the america's engine has any advantages performance wise over the SM i don't know not having ridden an america.
But either way you made a much wiser choice than going Japanese !
