I say UCL. USC is a great university, buts it mainly because of all the $$$ it recieves off of rich kids who couldn't really get into anywhere else (of course, that's a HUGE generalization, but this is the case in my school: if you haven't got the SAT scores, and you're a slacker, HK kids use connections to get in there. mainly just to party as well). Yeah, its got great professors and stuff and its ranked around the 30s in US, but, I mean, in terms of prestige or reputation, I think its nowhere near as good as UCL.
Philosophy and Econ is a very interesting course, so I think you should take that.