First of all, it depends on the encoder. There are many different MP3 and AAC encoders. The best of the AAC ones are iTunes, and the best of MP3 would be LAME encoders when using the --alt-presets(hydrogenaudio.org is your friend). VBR is a nice method of encoding.
In my experience, you don't really notice a difference with the higher bitrate stuff unless you listen to the lossy for a long time then hear the lossless. I tried this in a blind test, but it's really not much.
And, you'll notice it in the cymbals first and foremost. Don't bother listening for the rest if that song has cymbals - because if some other part of the music is screwed up a tiny bit with the lossy compression, the cymbals will probab ly be totally massacred anyway.
