Capo looks amazing, but if I'm going to drop consider dropping 40 bucks I want to compare :)
In Capo, you have all the functionality you need, with the limitation that after 5 minutes of audio pass through for a given song, audio quality will begin to degrade.
Learn a handful of songs with Capo, a handful with the others, and let me know (use the Feedback item in the Help menu) how it went!
ASD costs $30 and was a truly awesome app in its day. And it does seem to have all of Capo's functions, except for the nifty waveform display. But the UI is cluttered, overcomplicated, and ugly cross-platform stuff. If I didn't already own ASD I would just buy Capo. As one who does already own ASD I am still considering buying Capo.