Ezhikvtymane

Done both multiple times. Here's my honest take:

Oxford: more diverse architecture, the Ashmolean is world-class, the punting is more dramatic. The city feels more alive and less tourist-dominated in term time.

Cambridge: King's College Chapel is the single most beautiful building in England (fight me), the Backs are magical, the museums are excellent.

For a day trip: Cambridge. The King's College Chapel alone is worth the trip. For a weekend: Oxford. More to explore, better pubs.