It is a difficult one. Both Brown and Warwick are top universities and amongst the best institutions in the world.
First of all I think you need to consider what would be best for you on a more general scale; a UK undergraduate education or an American one? In the UK you are specialising, whereas in America you have to take general classes for at least a year I think before you streamline your course.
If you reall enjoy PPE and want to dive right in then I say think about that because in the UK you will be solely dedicated to the course whereas it will be different in the US.
Also think about the different cultures and climates and where you think you would be happiest at first. It's going to be 3/4 years of your life after all.
Second, and back to the universities, themselves. Well obviously, Brown is an Ivy League school which is the equivalent to the UK's Oxbridge and the big 5, so you might say Warwick is a teensy bit behind in prestige but not by much as the most famous Ivy schools are Harvard, Yale and Princeton I would say, Columbia too.
But, like I say, it is a tough decision definitely. Hmm...
On reputation alone, I think Brown would pip Warwick to the post so I'd head there. However, you do have to think about the different academic aspects and that things in the US are much more general, because do you really want to be fannying around with general classes?
On the whole, I've heard that undergrad educations are a lot better in the UK but postgraduate is better in the US.
Perhaps you should accept your offer at Warwick study for PPE and then do an MA at a top US school or something?
Good Luck!