Warwick is one of the premier maths departments in the country, along with Oxbridge and Imperial - seems to particularly have some strengths in pure maths. Bristol is pretty much the next closest contender to those departments for maths, although I'm not sure exactly how wide the gap between them is. Warwick is definitely in a tier above Bristol though so, look at each course format closely and think through your reasons for potentially rejecting it carefully if you do so.
That said for a few areas Bristol may be better within the maths sphere - logic and set theory for example I believe Bristol is stronger in than Warwick (at the research level anyway). I also get the impression for theoretical physics and related areas of applied mathematics to that Bristol might be a bit better, as Warwick seems to leave the theoretical physics stuff to the physics department.