Generally, Oxford is better known for the humanities and Cambridge for the sciences. If you're someone who believes in university ranking, it's always a better idea to look at the particular course you're interested in rather than university ranking as a whole. Also, consult your seniors to see if you're a good fit with the university, college, or tutor.

The other thing to consider is aesthetics - which city you prefer. While both are very beautiful with really old buildings, pretty architecture and amazing parks, Oxford is slightly more bustling than Cambridge and has a larger non-student population.

Further, there are some courses that are uniquely Oxford or Cambridge. PPE is one world famous course that you'll find at Oxford but not Cambridge, but if you're interested in only Philosophy, only Politics or only Economics, you can't get that here, but you can at Cambridge.