AAC is simply a more developed file format. Smaller and, to most people, better sound.
Like I said, if you want 40 gigs or even 60 fill your boots and buy a Zen but only if you're happy with the management software. Personally I simply dont have 10,000 peices of music that meet my standards of being 'keepers.' For me the whole point of these players, whether solid state or hard disk based, is I never again have to carry the fillers and also rans in a music collection.
Then again I only keep an average of three shots per 36 snaps I take because they transcend the ordinary and become a photograph.
You mileage may differ.
Regards
Rob