I applied to both on the assumption that the choice would be made for me, but then got an offer from both. I spent many a sleepness night trying to decide between the two (I actually got my other two choices too, but felt that London was for me), so this question is one that had run around in my mind quite a bit. In the end, only a few hours before the UCAS deadline, I decided that my heart would have to tell me the answer rather than my brain. I got a coin and allocated Imperial as heads, and UCL as tails. I decided I would stick with the coin's decision for an hour, and after an hour I'd see how I felt. It landed Imperial. After an hour I was still satisfied with this, so that was my decision. Never looked back. You could try this method!