What an objective answer....
With regards to CS departments, both are perceived very highly but ICL's CS courses tend to be more practical (like Psyk said) which may be better for you.
School prestige.. erm, both are very well respected and you can't go wrong with either.
Social life.. Imperial's full of internationals (mostly Asian) and a great portion of them tend to stick together and not fraternise with the enemy (non-internationals!), but then again, it's London and there are plenty of people and plenty of things to do, much more so than Oxford. Besides, you're international, so perhaps it might be easier for you to fit in.
Value for money.. well if somebody else is paying everything for you, then London. If somebody's gonna pay everything + a £180/week rent for a top room in central London, then I wouldn't pass that opportunity up.. Oxford is much cheaper, obviously.
Also depends which location you prefer, Oxford's smaller, more gloomy (aka strange buildings with domes and spires and medieval church like structures, I think the technical term they use to describe this is architecture
). Imperial is basically a block of buildings in South Kensington, with a massive erection in the middle (Queen's tower, altho the queen's not a man
).
Anyway, you'll be fine whichever one you go to. Just decide where you'd like to live for the next year or two.