You're on a speedmaster/america site, so you can guess what the bias of opinions will be. Obviously they're both great bikes and mechanically they're almost identical, so it comes down to a preference of style. The immediate thing that comes to mind is that the speedmasters are more comfortable on long distance rides. People on here who've ridden a bonneville (including myself) invariably love immediatley their nippy handling for the first fifteen minutes. Then their butts start complaining about the plank-like seat and they can't wait to get back on their own bike.
I'd suggest you take both for a decent ride and see how you feel about them.
Matt
