Assuming by "oxbridge degree" you mean a place on the course rather than a free degree certificate for no effort:
Definitely the money, for two reasons:
- there are plenty of other great universities out there, and just going to Oxbridge isn't going to earn me an extra £250k
- getting a good grade at Oxbridge is hard work - if you're the kind of person who effectively has to buy a place on the course rather than getting in for free in the usual admissions process, no chance you can pass the course/ get at 2:1, making the degree worth a lot less