My sister went to Exeter and im planning to go to Sussex. I had the same dilemma. I have spent vast amount of times in both unis and cities.
Its all about what kind of person you are. Sussex really does have a Bohemian feel to it, being next to Brighton it has the same liberal quirky feel to it. Staying with my friend in halls and meeting her friends i saw how the uni seems to attract interesting intelligent people who are all different and quirky (in a good way). There was one girl who even turned down a place at Durham for Sussex because she loved Brighton and the uni so much. None of them had a bad word to say about the uni or the city which is unique to allot of my other friends at uni. The night life is also one of the best ive been to better than many big cities (including Exeter). Brighton is also the place to be for a student, shopping is amazing with little vintage and independent stores to grab bargains and also all the usual high street chains.
Exeter is going to look marginally better for its prestige. It also has good nightlife allot better than many other cities. Exeter seems to attract (going from my sisters friends) more conservative upper class people, although im sure thats not the case for everyone but a bigger majority than Sussex. The city is also beautiful lovely buildings and little cafes.
I found what was really helpful in picking my unis is to try and see what kind of person the uni is trying to appeal to and work out if you fit that bill and if you get along with others who also fit that bill. see which one you are going to have the best time at because there is no point worrying about the 'prestige' because there both amazing unis.