In reply to holly:
Most of the hill colleges have a fairly similar atmosphere -- not too pretentious and generally quite nice, if maybe a bit lacking in character -- apart from Mary's, but that's now gone mixed so has probably changed a lot. Collingwood has the nicest accommodation of the lot.
The bailey colleges are a lot more eccentric, which can be good or bad depending on how you like it. Castle and Hatfield are both very rah-rah; John's is very religious; Chad's is religious and has a lot of 'confirmed batchelors.' All four of the above seem to have a lot of students who like to dress up in black tie and sing 'Jerusalem' very loudly in the small hours of the morning, or strip naked and run around public open spaces. Hild-Bede is nice, but a long way from all the others, and is a bit of a world of its own.
I went to Aidan's, and thoroughly enjoyed it. But this info is six years out of date.