Which would you buy & why?

Just a three parameters: class, wattage, cost are really too few. Plus, there is your preferences and speakers. Most speaker sound better with more power regardless of the volume you play them at… but then it depends on the implementation of the technology as to how it sounds. I would pick the one that weighed the most if there is a significant difference in the weight. No, I am not joking. The bigger the transformers the better… or if the did a big beefy case… both are very indirect but point to a manufacturer that is trying to put out a better product. Also, the higher cost would be in their favor as long as it was not a highly marketed product.

Class A for best fidelity, but no idea what speakers you're running. I believe in speakers first, then finding the right amp to pair with them.

A solid amplifier with a min 250-300 WPC/RMS!,

Buy the one that sounds best. 

There's more to it than using a low power amp and listening at low volume. The 20wpc has to be able to drive the speaker at low impedance. You may end up not hearing any bass at your low volume listening.

Need to know what the speakers are before choosing an amp.