IMHO: iTunes is (or at least was ... I stopped looking last year) pretty pathetic. Here's why. Suppose you want to listen to Bach suites for solo cello. Sure, they've got a version or two, but I want a version played by a master on a great instrument. Casals? Check, but old. Rostropovich? Nope, sorry.
Or, I want to listen to something (a lot) more current: Kronos Quartet? Some. Alarm will sound? ok. Bang on a Can? Nope, sorry.
It *does* seem to have both Glenn Gould recordings of Goldberg, which is an improvement (and, yes, they're very different).
This, especially the latter observation is surely connected to the recent /. discussion about use of computer-controlled instruments. It seems to have taken iTunes a very long time indeed to understand that two recordings of the same piece, by different ensembles or performers, using different instruments, under different circumstances, reveal the piece in entirely different ways. They're not the same thing.