The A was produced from '80 - 83 and the B from '83 to the ban. (I'm going from memory with the dates, so I could be wrong)
The B was designed is response to the ATF declaring that the A was to easlily converted to full auto. Apparently, it could easily be made to slamfire.
There are several changes, some of which don't have to do with the ATF's beef with the model A. I don't know all of them, but here are a couple.
- Sight differences. (A=Adjustable with special tool, B=Adjustable without tool)
- The bolt on the B has a lever that must be mechanically depressed in order for the striker/firing pin to reach the cartridge. This is depressed only when the bolt is in battery. This satisfies the ATF's beef.
- A guide on the right rear is welded on in the A, whereas it is a part of the receiver on the B.
- I believe the trigger group is slightly different as well.
The only difference you will detect in shooting the two is that the B has sights that can be readily adjusted. The A has the SMG sights.
The A is more desirable, but not so much as to turn down a B. They are simply not that different in doing what they are designed to do. It's simply better to have the one that the "nanny state" required altered.