The commercial version is definately worth the $19.90 (including shipping).
There's much more music, better controller support, unlockable content (with points you gain after completing a song (more points for the better you score on the lavel)), and overall, it's a very professional piece of homebrew work.
Many thanks to Dan Potter and Roddy Toomim (aka DJ Geki) for setting up and maintaining a great, informative, fun message board before the official release of the game.
Great game, I believe I've plugged it more than once on this site already, and I hope I've convinced some people to go buy it. It's money well spent.