The bb5 has a little less leaverage than the bb7, they also do not have the adjustability of the bb7. If you are on a budget get a bb5 with a half decent lever. I have a cannondale lever on the b221 with a bb5 and it locks up easily, however the echo has a bb7 with a speed dial 7 by avid and I can adjust how the brakes work (feel and modulation). I prefer the bb7above all other cable brakes, DO NOT get Hayes they are crap!! mx1 work when they want to and the mx2 have no adjustability besides 1 pad adjustment.
mx1 1 adjustment
mx2 1 adjustment
bb5 2 adjustments (both pads)
bb7 5 adjustments (both pads, 2 cable adjusters, and a modulation adjuster(so you can use a cheepie lever))
The best brake out there is the hope mono mini trials brake (get a second mortgage to buy these)
Hope this helps
Jason H