I'm assuming you're meaning things bought from the iTunes store v. things streamed from Pandora. Otherwise iTunes just plays whatever files you have, assuming it's of a type it supports (ie mostly it can't be flac)
Pandora's files on the free account are relatively poorly encoded 128kbps .mp3(sometimes I think they're actually less, they just show up as 128kbps). The paid version 192kbps .mp3.
iTunes store songs are 256kbps AAC, which I feel is equivlanet to about 320 kbps .mp3. That is to say, they are a good bit better than even the premium account pandora songs.