Well obviously the small amp is a class A and most likely rated at 25x2. With only 9 amps of fusing and being a class A it's not going to be a power house that's for sure...lol
Class A amps are not known for efficiency but are known for high fidelity. A theoretical efficiency of 50% is obtainable with inductive output coupling and only 25% with capacitive coupling.
Selling prices are all over the place... check eBay sold listings. The "Class A" isn't true class A as (I think) no car audio amps are. It does have a slightly higher bias and this does seem to make a difference. The Class A amps are a tiny bit smoother on the high frequencies. The 502 is a good amp too.