I have chosen to apply to Cambridge (for Computer Science) because it is virtually the birthplace of many aspects of computing; Turing and Babbage also went there. It also looks very green and it seems like just the right size as a city. The Computer Science faculty building also looks so much better than the dull Oxford one, and the Colleges at Oxford all look a bit bland to me.
Also, my wonderful history teacher (who walked out of Oxford with a 1st, no less, and went on to make a certain website that a lot of history students use) said, and I quote: "Oxford is a bloody dump once you leave the centre." (And he should know: he comes from Telford.)
Edit: My comments were purely tongue-in-cheek, you touchy Oxonians; I would rather go to Oxford than anywhere else other than Cambridge. Also, Telford (probably) isn't as bad as he makes it seem...