Just a few remarks:
--> Final grades aren't everything, but they make life so much easier, both concerning career prospects and self-confidence. Hence you should choose the university, who seems the best for you! Supervision may be very beneficial for one student and for the other totally not. There is no general answer.
--> available specialisation: towards the end of the degree, you can choose between courses, how much that depends on uni. While your favorite subjects can change with time, you should look, that the university course you choose, actually offers courses in your favorite field.
--> year in the industry: experience is very important and helpful, on the other hand it might be also possible to do longer internships at the other universities
--> as a lot have said Imperial would have no social life: The students I met, were all really happy with it. But one thing is sure: Engineering is not the subject, where you get to spend lots of time out, but that is down to the work load, has nothing to do with the university.
--> Generally look at grading, course structure, course content ... also look at what you think is essential for you to succeed and whether that is available at university
Concerning reputation: As said already, Imperial and Cambridge are considered equal for engineering. Hence either way, one would go to a very prestigious university. (Which would not meant, they would be the perfect choice.)