I use both, and BB cuts out a LOT of crap before it ever gets to SK2, or the registration page for that matter.
For me, SK2 is more of a back-up to BB. BB isn't perfect, but SK2 isn't either. However, if you've ever seen a "Swiss Cheese" model, you'll get the idea.
Although Quentin will bark at me for it, I also use a 3rd option. I have a captcha on all comment forms, and the registration page as well.
A 4th gimmick on the registration page is a checkbox, that is rendered with Javascript. Initially the submit button is turned off, and if JS isn't enabled, it can't be turned on because it's turned on when the checkbox is ticked.
It isn't perfect by any means, but all combined it cut out splogs by 98% or so. As a note, SK2 doesn't have an effect on the registration page.