The B3 is a 3 port engine. It has a lot of torque and will pull you out of the turns really hard but kind of lack top end. The B5 is a 5 port, it has almost as much bottom end grunt as a 3 port and almost as much top end as a 7 port. I think the B5 has better fuel mileage than either the 3 or the 7. It has balls down low and doesn't appear to struggle at all in the top speed department.
The B5 is a killer deal. $160 for a B3 is a sweet deal as well and I may pick one up in the future but for an all-around race motor I chose the B5.