power cable: more strands, or bigger strands?
If the inside wires themselves are not individually insulated it will make no difference. 

If they each have their own insulation, it would be called litz wire, and then the only difference will occur at high frequencies due to the skin effect and the proximity effect. I don't think your inverter application will achieve frequencies that make any difference here. If you were using it for such a thing, the 20awg individually insulated wires would be fine until ~8KHz and the 100 individually insulated super little ones would probably be fine into the 1MHz range, it depends on its diameter.

So basically, whichever one you like. Flip a coin.