I use Google Voice, mostly because it's free when calling phone lines.  The audio quality appears to be about the same, and I otherwise find them indistinguishable.

Skype does do desktop sharing, which I do quite frequently on ReckonLabs business.  But that's a special case, and not enough to tip my vote.

I did once have a collection of Skype credit.  Then it expired.  Apparently the minutes went bad.  Because they spoil, you know, if you don't keep them in a cool, dry place.

Am I the only one who doesn't like staring at the face of the person I'm talking to? (And in turn having them stare back at me?)  I find the whole idea very uncomfortable, and a bit of an overshare.