I don't know much about Exeter, but for the longest time Cardiff was where I wanted to go. The course seems really really good to me, but perhaps that's just because of my interests (they do a lot of number theory, and also offer modules on cryptography and the foundations of computer programming languages which I find interesting). In the end I didn't take it because I didn't want to go to a city centre type University and they didn't offer a 4 year option. I think there's a lot of difference between maths degrees at different institutions though, so I'd probably try to factor that into your decision if you know what you want out of the course. Exeter seems pretty solid from looking over their website.