I had a '99 and an '01 Blackbird. Here are a couple of observations:
Powertrain
Not quite as silky-smooth as the XX, but almost. Minor buzz @ 5.5K, nothing serious. More power and torque everywhere, slightly better throttle response than the XX. Transmission shifting feel is similar, XX slightly less positive.
Brakes
Big difference. The Busa could use better brakes, but still stops much better than the XX. My wife and I do a lot of "Cannonballing", and the XX definitely faded more than the Busa. Busa "feel" slightly better.
Chassis / suspension
The XX is slightly plusher, but at a cost (the fork dive was especially annoying). The Busa feels lighter and tighter than the weight difference would suggest (the XX is significantly heavier than the Busa). It's easier to make mid-turn corrections on the Busa tham the XX without upsetting the bike.
Comfort
This is very subjective, but I thought the comfort was about the same overall, with a slight nod going to the XX for it's broader seat. Rider geometry is similar between the two bikes. The 2001 - XX has a taller windshield than the Busa, almost a factory "Sport-touring" windshield.
Quality / Finish
The XX has a bit of an advantage here. The finish looked better on my '99 XX after 25k miles than my '01 Busa at 7K. I have also had some mechanical problems with my Busa (cam chain tensioner and some electrical problems) than I didn't have on my XXs.
Overall
It isn't far from the truth to say the Busa is a faster, better handling XX. There is a noticeable difference in performance, with little sacrifice in comfort.
I think almost anyone coming from an XX would enjoy a Busa. It is one of the few bikes that I have ever had that met my expectations.
Good luck with your decision,
Mike