The colleges in Cambridge and Oxford house you, deal with pastoral stuff, are where many of your friends are etc. They supervise your studies, arrange for supervisions, enter you in exams etc.
The university or faculty provides the lectures, sets the course, organises exams etc
So you get pretty much the same level of teaching at all the colleges because its mostly done by university, and many of your supervisions will be cross collegiate (so they send you to a grad student who is studying your topic who may be at another college)
The college is more of a social, practical and asthetic choice. Do you want to live near the river, in town, in an old building, new building, etc etc.