Essentially nothing, unless your going to buy second hand.
The AM-1b is a revision of the AM-1 (just like the HC-2b is a revision of the HC-2) and while Ring, retailers etc just call it the AM-1 you can't buy the old AM-1 new anymore, except perhaps if a retailer has some exceptionally old stock you'd be getting an AM-1b.
The difference was that the old AM-1 was basically just a turnout controller whereas the newer AM-1b can be a turnout controller or a proper accessory module that you can write programs for.
- Tim