I think it's worth mentioning - since it wasn't really stressed enough to me - that if you want to apply to Cambridge for economics, really exceptional UMS scores will benefit you hugely. I know people might say 'Oh, but my friend got in with A*AA' or something else, and there's no denying that that must happen, but you have to realise that it's an intensely competitive subject and they ask for UMS scores for a reason. I think when people quote anecdotes like that they're trying to convince themselves that UMS scores aren't that important. You're competing against people from all over the world so of course the better your grades the better your chances. Other people will say 'The interview's also important', which it is, but if you want a really fair chance then you'll also need excellent grades.
It sounds really harsh for me to say all of this, but bear in mind that I was rejected this year after months of awful stress and anxiety about preparation and reading and applying to UCAS early, etc etc. I just wish now that I hadn't bothered, because it made my January modules suffer (not amazingly, but I stopped caring about work after doing so much for no reason, now I have to re-take them to keep my options open).
If I could go into the past I'd probably have applied to Oxford, if I wanted to apply to either of them. It seems a lot more laid back, you get several interviews so a catastrophe in one of them isn't as important, you don't tell them your UMS and you don't have to fill in the stupid SAQ (supplementary application questionnaire) which is a complete pain in the arse and you have to take pictures, give class sizes, and tell them topics you've done at A-level (even in subjects like English that are completely unrelated to economics). It took me weeks just to get round to doing that.
The conclusion of my rambling heap of prose? If your UMS aren't that amazing, apply to Oxford. If they are amazing, apply to Cambridge. Even though it's still possible at Cambridge with average UMS scores, you want to maximise your chances. Unless, of course, you're adamant on one over the other. You do get 5 choices, after all. Anyway, good luck guys!
EDIT: One last thing - think about all 5 choices really carefully if you're meeting the early deadline for Oxbridge. I ended up just sticking down some unis that now I don't really like the look of because I just wanted it all finished by the deadline (and my school's UCAS person was a right termagant). Remember that Oxbridge is always going to be a long shot and you need some other unis that you actually like.
And, for information, I was predicted A*A*A* with low 90s AS averages, and that was probably reasonable for Cambridge. I don't have statistics to reinforce that point but I know a lot of people had a lot better. And please don't take all of this in the wrong light - I just don't want people doing what I did and wasting months on Cambridge.