Depends what you want from your student life but id think Bristol would be a more fun for typical student atmosphere, more clubs, bigger city (but not too big either), 2 universities so bigger student population, more bars/restaurants/cafes.
Exeter is quite a small place, minimal nighttime economy students rely more on house parties and things I think but supposed to have a nice campus.
I dont see why Bristol uni would have significantly more drug use then Exeter uni although everyone has heard that reputation, ultimately students will be students wherever you go, some people will take drugs at every university, just dont take them and if you dont want to socialise with people that do then that's fine as well (Exeter has 23,000 student, Bristol has 27,000 you'll encounter people who make different choices whichever university you choose).