In terms of league tables they are only going to be a few places apart, both have good UK reputations. Imperial would probably have a greater international reputation, better for further study prospects particularly abroad whereas I think Durham always has had an excellent employment record in the UK for all its courses.
Imperial is in London so it is bound to be more expensive, this may be a problem, particularly if you have to rent out your own accomodation for a year. On the other hand, many people like the bright lights of the city.
Easiest way of looking at it is city v town, Durham is very small I think but it looks lovely - I will probably be going there next year, I live in london right now and want to go somewhere quieter.
Above any employment, teaching, research statistics (you're going to get a good degree and good job whereever you go if you're bright enough), just think about where you'll enjoy yourself most.
By the way, what college have you applied for at Durham? Ive got offer from st chads.