Sushi,

Assuming your uses were similar to dwm's, and then aac would stend ill be a poor choice compared to MP3 as it's not that well supported at the moment. You're basically relegated to the iPod and computers plus there is the limitation of DRM (not as bad as most but not as good as ogg or MP3 either). Sound quality comparisons I've read seem to all indicate that aac falls inbetween ogg and MP3.