Unless there is a deposit to pay, then you can accept Oxford and reject it later when you get the Cam offer (not biased, but ...)
The Colleges are quite different. St Anthony's is a modern college, which usually also means poor, but I don't know if that's true in this case. Unusually it has a bit of a subject specialisation in IR/IS/regional, so it may be academically stronger for Dev Studs, again, not sure. Newnham is not an ancient College, but far more established than St Anthony. It has pretty decent funding and a good rep in Cam for having a strong community. It has no subject specialism. I don't know what the general rep between the two it, but Dev Studs seems to have a decent rep at Cam.
The pressure of a 1 year programme at Cam isn't that bad, still plenty of time to have fun.