I think there are only Model A ($25) / B ($35), and rev. 1 / 2, there is no rev. B. Most RPis available on the market now are Model B rev. 2. I don’t know where you can get Model A in the US. I believe the difference between rev. 1 and 2 is that rev. 2 has two screw holes on board which rev. 1 doesn’t have.