Imperial prides itself on being very technical and practical. It carries the reputation of being science and engineering orientated.
UCL is excellent in these fields too but takes more pride in the arts and medicine to some degree.
So in the long term it is about what you would rather be associated with. Either way UCL still has top class science and engineering departments, imperial is just more specialised while UCL has a broad spectrum (this has pros and cons).
But, I consider UCL more international, it has a vast network. And it is by far more enjoyable. I know I would have had a greater experience had I gone there.
Honestly with these two you are not losing out on much in terms of opportunity and academia. Students too often disregard travel, accommodation, finances, the local area and just the general activities. These factors really influence your life at university to the point it can affect your grades as I was working to finance myself and travelling hours each day.