Imperial isn't as fully science based as people make out. They also teach a variety of technology degrees (no. 1 in the world for this), business (the Tanaka Business school) and they offer additional humanities courses for students if they wish to take them, with extra marks available for students who do. Still, you will probably find you spend most of your time with medics just like at all other med schools, even if you come intending to make sure you have variety in friends.
As for which one to pick, Imperial would probably be a good choice. You'd probably enjoy the change from the environment you've been in for 3 years, especially considering you're looking at the next 6 years of your life. Imperial also takes you to hospitals in NW london with very good reputation, such as Charing Cross, Hammersmith and St Mary's. With regards to jobs at the end of med school, as long as you do very well, you can probably get in anywhere you want from almost any medical school. Still, you are more likely to make the connections and get jobs at the Imperial hospitals if you go to Imperial.
UCL also has nice hospitals like the University College hospital but I don't know much about them. At the end of the day, it's your choice; Imperial is consistently ranked higher than UCL in the league tables for both medicine and overall

.