The JB is Duncan's #1 selling pickup. Amazingly enough, it seems like most people here find it to be a little to aggressive in the upper mids/treble. It does sound cool when turned down a bit...kinda Tele like. It's a pickup that always impresses the heck out of me when I hear it again after not having heard one in a while, but then I get tired of it and it seems to hurt my ears. Weird!
It's Seymour's favorite humbucker but he uses it with 250K pots which bleed off some of the treble even when on "10". I tried that but it made the JB a little dull sounding to my ears.
Anyways: I'd go for the Custom.