The 256kps that Apple offers is slightly superior in quailty to the 256kps MP3's that Amazon offers. They are both VBR so the size of the files are comparable.
However, you got to believe that the slightly superior quality of the AAC files is FAR offset by the 30% increase in cost for most new / popular songs that iTunes is now charging over Amazon. Wait - most songs on Amazon are oly 89cents, not 99 cents, so that's even greater (40%). And if you buy as much music as I do, 40 cents difference per song is VERY significant. That a $40 difference for only 100 songs (10 CD's or less). Note that it's just as EASY to purchase from Amazon, as it loads the songs directly into iTunes without any user intervention, as does iTunes.
There's really no reason not to buy from Amazon at this point. It used to be that iTunes offered superior SELECTION, but I've heard that they've basically pulled everything out of the store for which they could not get DRM-free approval for, so even the selection is much worse now. Higher prices and worse selection..........Hmmmm.........
Tony