I think that Kent is on the whole the far more prestigious institution. That's not to devalue Hull at all, it just seems to me that Kent is just a higher tier institution on the whole. The University league tables tell you a lot of what you need to know, though please do take these with a pinch of salt. But the consensus seems to be that as an institution, Kent ranks significantly higher than Hull. Kent in the Guardian guide is the 23rd highest ranked in the UK, in the complete university guide it is the 22nd, as a general institution. Hull is 71st on the Guardian table and 68th in the complete university guide. Now as it happens, when it comes down to looking at the music courses offered at undergraduate level, they seem much closer.
However, on the whole, generally Kent will sound more impressive than Hull.
As for location, it so happens I've visited both in recent years and even have a friend who studies at Medway. Whilst the advice above has some truth to it (certainly Canterbury is in my opinion much nicer than Medway) I think there are -worlds- between Medway and Hull as places to live. Hull is quite dilapidated, pretty far to the North, as a result not privy to the same sort of transport network that being a stones throw from London allows you. Hull is seen as quite a rough Northern city. Medway by contrast is seen as fairly rough, worth noting though is it appears to have significant funds for regeneration over the last few years and is getting far nicer. I'm reliably informed that the campus by the dockyards is actually rather lovely. Whilst I'm not going to pretend that the area doesn't have it's issues, as it does. It is a stone's throw from London by train, with good links to the rest of the UK as a result.