| Both are Top-loading bridges and you can choose between brass and aluminium. The main difference as joeyl mentioned before is that the A offers string spacing adjustability and the B doesn't, but you can choose between 3 different string spacing templates when ordering one. The A model also offers a string through body option.
I own the A bridge, I really like how it looks on my J-bass and the adjustabilty allowed me to have excellent intonation, string spacing and action. It's really heavy though so if you're looking for a light bridge it wouldn't be a good option. | 
