I've got the BB5's on my 04 FSR xc. My friend sprung for the BB7s on his 04 FSR xc. Stopping power is basically the same... The initial set-up is pretty even. However, once your pads start to wear, the BB7 is very easy to quickly fine tune (without having to preload the brake cable). The BB5 requries you to adjust the barrel adjusters to move the outboard pad in...
Also, the BB7 uses the same brake pad as the rest of the Avid line, so you'll have more brake pad choices. The BB5 uses its own brake pad, and you're limited to either Avid or Koolstop branded pads... (For me, it's a little annoying but not a huge deal).