The B engine has a bigger intake port than the A engine. I've been told that the port timing between the two is the same, but the port area between the actual port window and carburetor mounting flange is bigger on the B version. I've got several of both engines and I've been going to measure the port heights of an A and a B just to verify that they are the same, but haven't done it yet. As far as I know this is the only difference between the two. As far as value, guys that are building one lunger racers for the stock class would probably prefer the B model, as they're rated at 22hp. The A model is 20hp. But if someone is restoring a sled or if they're building a mod class racer that's going to be ported anyhow, I'd say their isn't much difference in dollar value between the two engines, IMHO.
I race with hex clutches....