I go to Exeter Uni but live in Cardiff so I have a decent experience of both places.
I'm not going to try and brag about Exeter's nightlife, it is definitely not on the same level as Cardiff. Exeter has a few gigs but Cardiff is really gonna have a lot more events.
I had Cardiff as my insurance but didn't really want to go there, I didn't like the "campus" at all, since a lot of it is split over the city I didn't really get a "Uni" feel to it.
Honestly though, if you did go to Exeter and wanted to go to some Cardiff Events, you can get a train ticket Exeter to Cardiff for under £20 if you book in advance.
There are also a lot of people in your predicament who will have gone to Exeter still - I don't think you'd be the only person who was one of the grungy kids haha
Hope I helped anyway