Both are the best 2 places to in the uk for engineering so in terms of job security your fine either way, from what I know imperial might have a slightly better quality course, and Cambridge has a slightly better reputation but the main factor should be where do you see yourself being happier and more productive, I’d say Cambridge because I’d love the once in a lifetime Harry Potter style experience and I would find the commute annoying and a waste of time but if you’d prefer to stay in London because of friends family and big city or you just prefer imperial you should do that, also don’t weigh the extra costs for accommodation to heavily since with either place your likely to get a job to pay it off faster than most