The differences between 256kbps and 320kbps is really negligible. The quality of the compressor (i.e., FFMPEG vs Fraunhofer) is more important at that high level of bitrate -- assuming a standard 44.1kHz stereo music file.
VBR vs CBR is also more important. While VBR should be better in theory, it doesn't play nice with playback hardware or software, so always choose CBR for compatibility's sake.
sells most of its music as 256kbps, as does iTunes -- at least last that I paid attention. I've not bought an MP3 in a few months now.
Most people who would insist 320 sounds better than 256 are likely overlooking lots of secondary factors. They're just jumping to conclusions without actually analyzing all possible variables, and scientifically forming theories based on controlled experiements (i.e., the scientific method, which most people are taught in junior high school).
and iTunes have decided that 256kbps CBR is a good balance of quality/size/compatibility, then I think it's safe to assume it is.
It's been a while since you've posted. Hope you're well.