Hello!
At first, the two parent institutions are world class universities, so both of them are excellent. Here is my advice:
In terms of Business & Management, I would say Imperial would be better choice than UCL as their Business School's reputation is increasing sharply and their placements in the finance and consulting sectors are among the best. In terms of curriculum and layout, Imperial offers a wide variety of management programs which results in an impressive range of electives, projects and study abroad opportunities. Also their rankings are very good (if rankings matter to you).
Finally, one of the main differences (which depends on what you like you best), UCL is going to be very theory-focused whereas Imperial is going to be more practical/career focused.
To give you a global vision, in terms of business good business schools in the UK, this is my ranking:
Tier 1: LBS/Imperial/LSE (not in order)
Tier 2: UCL/King's/Warwick (in order)
Tier 3: Cass/Edinburgh/Manchester (not in order)
Tier 4: Durham/Bath/etc.. (not in order)
Considering this, what would you prefer? Where would you think you would be the most successful? Where would you fit the most in terms of career clubs/extra-curricular? I think that GPA and involvement matter, so you need to chose the masters where you would make the best of it, this is the most important.
Finally, do you have offers from both schools?
Hope it helps,
Cheers!