As for how I choose my own song list - Masked Man has pretty much nailed it above.
Here's how I judge A vs B:
First, I look through and see if it's an obvious win. If A has only one song I know and love and B has six, then I'll call for B. But if they seem at all close, I bring a point system into play - might not be the best (none of us has time to download and evaluate all the tracks we don't know), but it works for me...
For songs I know: I give three points to one I like a lot; two points to a song I like quite a lot; one to a song I think is okay. Nothing for a song I don't like.
I don't like to give zeros to songs I don't know, so I award one or two points depending on whether I know and like the artist. If it's an artist I consistently don't like I might give 0. If I don't know the song and know little or nothing about the artist, I award a neutral 1.
Anything over 20 is a good list.