It's going to depend on how transportable you want those files. If you want them to play on almost everything, MP3 is your best bet. Maybe choose something like 320kbps. That's always seemed to work fine. Then you can play it on any MP3 player, PC, Mac, PS3, Tivo, etc.
If you do FLAC, you're limited to only a few portables. If you do AAC, then it's only Apple. If you want music from the Apple store, it's only Apple products for playback, can't use Sonos for example.
I've started to move to lossless but since most of my equipment is Apple, I've gone w/ AAC. The guys are right, hard drives are cheap so don't make decisions on that.
