Before I purchased, I looked at and test rode just about everything out there. When I sat on the Sportster, I noticed two things.. First, it felt a bit small to me (I'm 6 foot and 225lbs for reference), and secondly, the fit and finish just didn't seem up to par. It looked and felt like a cheaply built bike. In contrast, the build quality of the Triumph was far superior, at least to my eye.
As far as performance.. I dunno. I'll go with Rons assertion that they are about equal in performance going down a straight track. That doesn't matter much to me, as I can get to the speed limit plenty fast; and being able to do it a second faster or slower doesn't really matter all that much unless you are racing! But the real difference in performance, and one nobody seems to have mentioned, is in the curves. On twisty back country roads, my Triumph eats Harleys for lunch. In fact, I'll go so far as to say that the Triumph even handles the curves better than my Ninja.
One caveat though.. my Triumph is a Speedy, not an America, so while basically the same bikes, the riding position is slightly different; the drag bars and lower riding position may give it a slight advantage over the America in that respect.
Cheers,
Brad
