In general, they say Oxford is a city with a university in it, whereas Cambridge feels more like a university with a city around it (something like that anyway).
There are 30-odd colleges at both and it can seem bewildering. But to start with at C -
St Edmunds, Hughes Hall and Darwin's are postgrad colleges so they're out.
Newnham and Murray Edwards are female-only, which may or may not appeal.
Churchill (quite a STEM-focused college), Fitzwilliam, Robinson are colleges with 60s / 70s architecture which may or may not be an factor for some.
The smallest colleges (in terms of student numbers) are Peterhouse, Corpus Christi and Trinity Hall. Could mean a very supportive / bonding experience, could be a bit claustrophobic?
The largest colleges (I think) are Trinity, Jesus, St Johns and Homerton.
Girton is quite far out of town (by Cambridge standards) - about 2 miles out to the north of the centre. Homerton is about 1.5 miles to the south of town. Both have huge grounds. Homerton has an orchard, beautiful new dining hall, accommodation on site for all three years. It is the most diverse college in terms of student intake and has a reputation for being very friendly and forward-thinking. Girton has a pool. It used to be a woman-only college, so a lot of history about inspiring women all around.
Kings is the only college who do not wear gowns to formals.
Emanuel have horrible first-year rooms apparently, but the good news is they are building brand new accommodation for next year, so could be worth a look.
DC informs me he is glad he's not at Christs as they close their bar during exams!
I have also heard that at St Johns, choice of second year accommodation is determined by who does best in first-year exams.
But in general, all the old colleges are stunning and even the modern ones have beautiful grounds. You can't go wrong with any of them.