DS2 liked Oxford when he visited last summer and likes the structure of the Physics course there.
For comparison, he has gone to Cambridge today for an open day at one of the colleges. As well as the opportunity to look around the college, there will be some subject-specific talks (Natural Sciences: Physical).
DS2 wants to apply for a Physics summer school which takes place in the first week of July. If he is lucky enough to win a place, he will not be able to attend the summer open days at Oxford or Cambridge.
As far as I'm concerned, the purpose of today is for DS2 to rule out applying for Natural Sciences at Cambridge and exclude it from his shortlist. If he comes home inspired and enthralled this will not make things any easier.
So far, wherever he has visited most recently is the one he is most impressed by.