Which would you buy & why?

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.

Buy the one that sounds best. 

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

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.

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.

Who can say?

What speakers are you running and what is their efficiency? 25wpc is enough power for speakers with a 93db or more efficient. The 70wpc is a safer bet and it will not have the heat problems that a class A amp might present.
FWIW I agree with you about low wattage amps even though I play my music somewhat loud. But it doesn't take that many watts IF your speakers are efficient.