I'd say they're not too far apart. The GT is a different story. The Buell will be much quicker handling of course!
They seem pretty different to me. One is a fully faired sport tourer and the other is a mostly naked backroad machine. One is a 500 lbs. and 90 inches long and the other is 375 lbs. and 78 inches long. To me they are two completely different mototrcycles designed for different purposes. I suppose if you were a masochist, you could tour on a Buell, but I think that is exactly why the BMW was produced. Shaft drive, heavy and long. The Buell is more of a race replica.