Amazon has double the bit rate though. That may not matter to some people, but I'm not going to spend money on 128kps songs. I'm no audiophile, but there is a difference at that low of a bitrate.
As of April 2007, DRM-free iTunes songs are 256kbps AAC. Amazon MP3 is 320kbps MP3. Audio file formats are a highly-debated and subjective matter, but the consensus seems to be that “AAC is higher quality at the same bit rate, so you can use a smaller file to achieve the same quality as MP3” [1]. (Originally DRM-free songs on iTunes cost extra, but now they are the default.) [1] http://www.planetofsoundonline.com/art…
News to me, thank you. I skimmed Wikipedia to double check my figures, but I misread it. In either case, I still prefer MP3 to AAC (though converting is trivial).