Lots of people ask "which is better" which isn't a good question...The reason being that in different circumstances one or the other would be better. The B1 is a cardoid, and the B3 has three mic patterns (omni, figure-8 and Cardoid I believe) that's the major difference, and it means the mics sound slightly different. I don't have the mics so I can't say what that difference is.
I would suggest you ask for soundclips of the mics in action, or get a pair loaned to you and test them. Then you'll know which is better for YOU. That's the important part.
Jacob
