Oxford is, of course, better.
You know I'm being facetious, right? They're basically unsplittable quality-wise and have been forever. And they have a LOT of similarities. But there are reasons why one might be better than the other for you:
They don't offer all the same subjects. If you want to do just a physics or chemistry degree, forget Cambridge. If you want to be a vet or a dentist, forget Oxford.
The exam system is different (Cambridge has exams in all three years, Oxford doesn't).
There's a different emphasis put on things in the entry system. You're more likely to get an interview at Cambridge. You're also more likely to get a really high offer and fail to reach it - almost all Oxford offer-holders achieve their offers but a significant proportion of Cambridge offer-holders don't.
Cambridge has a pool system which goes on for ages while you'll know whether you've been offered a place at Oxford on the same day as everyone else in mid January. You can either look at this as keeping you in the system for longer (at Cambridge) or knowing earlier where you stand (at Oxford).
Oxford is a significantly bigger city than Cambridge.
Oxford no longer has any female-only colleges.
It's not worth setting your heart on a college - at Oxford about 40% of people end up at a college they didn't apply to. If you have a preference, great, but if not then it really doesn't matter.