I can't help you much with info about Essex, but I've just finished my first year in politics at Leics and love it. In the first year, IR and Politics students study the same modules anyway, so the two departments are very close. I don't think you should base your decision on rankings though, I think you should perhaps look at the module choices in the second and third years, and see which you'd be most interested in.
Also, visiting the campus, and researching the surrounding areas would be a good idea. Remember, you're picking a place to live, not just the university itself.