I would choose Imperial over Warwick for any subject EXCEPT Maths. I've heard very negative things about the support offered to Imperial mathematicians- they're pretty much thrown in and left to fend for themselves, struggling to obtain the 2:1 they need. IIRC there is also a lot less choice available in the Imperial course (I might be wrong about this though, so check). As a result, despite Warwick's Maths course being similar/slightly harder compared to Imperial Maths, and the students being of a near-identical calibre, the exam results for Warwick Maths are actually a lot better.