I have had both...at the same time....An 01 busa and an 07 R1.
The busa, imo is def a bike for the more experienced. The R1 has a throttle restriction in low RPMs and does not allow you to grip it and rip it in low RPMs. You can still wheelie it, but your chances of an inadvertent wheelie are very low because of restriction. The R1 is also a lot lighter and when your turning, it wants to push you straight up out of turns, so your turning recovery time is faster.
The busa feels alot more unrestricted, you can wheelie off the line, and its a lot heaver and lower. Especially if you get a corbin seat, you feel like your "inside" the bike than sitting right on top of it. When you corner, you dont need to throw much weight into the turn (unless its a hard turn). It feels like the bike is getting down for you, but there is more work on recovery when turning in your body weight.
What it comes down to is that there is 2 different riding styles, but if your looking for "the next step", from experience i would go with the R1 before the busa. I had my busa before I had my R1 and I feel like I would have been a much better rider on the busa had I had the R1 first.