Thanks for the responses, and sorry to be slow to reply.
You're right, it's best to make the decision on an academic basis, but this is difficult to gauge as both departments are excellent in the field. The Oxford supervisor is an authority on the subject, but he's been out of the country all year, so I haven't been able to discuss the proposal with him. The tutor at Cambridge is also an authority and I've had more success corresponding with him; he sent me an email shortly before the AHRB deadline encouraging me to direct it to Cambridge. Difficult to ignore advice like that! However, Worcester is very tempting still - especially since I live in Oxford already.
Questions about Worcester: the secretary tells me they are over-subscribed for accommodation so I am not guaranteed a room, but I hear almost all first years get a room in the end - is this the case? And whereabouts are they located - any rooms in the older college buildings? Are they nice? I'd be very interested to hear about the MCR - the number of people, general social life - and also the quality of food, the bar etc.
Thank you very much!