As a person who applied to both UK and US and currently still hold an offer to a uni in both, I can tell you that I am somewhat biased towards the US.
I think that in general, the top 20 US universities do better in research than the top 20 universities in the UK. This is mostly due to the fact that there is much more funding for research in the US than the UK, and the US is able to attract a higher calibre of graduate students because they are able to give more financial aid to students than the UK.
However, comparing USC to Imperial is quite hard. In this situation, I would say that an Imperial degree is much more prestigious than a USC degree, but in terms of teaching/learning it is quite hard to say which is better.
In terms of social life, Imperial has all the city life you want. USC, on the other hand, boasts a big beautiful campus which Imperial will never be able to offer. You could also go to the beach while at USC, for example whereas that would be hard to do in London. Also, LA is a pretty nice city on its own, and will probably provide you with all the stuff you need for 4 years.
The US and the UK are in different worlds. You yourself need to decide what you want academically, socially, and what is best for your future.