You will definitely want a lossless format for use on your Axioms. The difference becomes quiet noticeable.

AAC is probably the worst choice unless you absolutely need it to play on an iPod. I maintain a parallel library of FLAC for my home theater and 256 MP3 for my portable. it's really not that much extra work if you have to re-rip all your CDs anyways.

With great power comes Awesome irresponsibility.