I'm at Essex university, obviously not studying Politics but I'd say it completely depends on you.
Both universities are very different, Essex is a very, very campus based university. Notts is more of city place, you'll get two very different feels.
As for Notts being Russell Group, Essex is pro-1994, which is on an equal ground. So put them both on an equal level as far as "rankings" go, though of course I can't say that Notts will be higher regarded, but definitely not to an extent to which an employer is going to go "no." If you are worried about ratings (which many post-graduates will tell you and employers themselves, unless it's Oxbridge, they will not care)
However, Colchester is not a very "political" place. Nottingham is a big city, depending on what you want to do in life I'd say go there for a better chance of making connections and if you like the city and could see yourself living there, I fully recommend going there. But again, I don't really fancy living in Colchester by any means or working there so that wasn't a factor at all for me. Hell, I want to get as far away from England as I can as soon as I graduate.
Look at the course modules, go to the unis and see which one you feel much more at home at. I had a choice between Leicester and Essex for history, Leicester being seen as much better as a uni but I felt so much more at home at Essex. I loved the campus and I still do, if I had the choice between both I wouldn't have to think twice about choosing Essex again.
Everyone at Essex and Notts will probably try to promote their university as much as they can. But in employment, the name of the university you attend will only get you so far and Essex and Notts will not be enough to put you ahead of anyone else regardless of what people may say. You presentation, what you learn in your degree, work experience etc. all depend on what you do at the university and if you enjoy the university more, you will put more effort into doing more work experience related things.
tl;dr - Name of university mean nothing in the real world unless it's Oxbridge, Notts if RG and Essex is pro-1994 which both mean nothing for UG, decide upon the course modules you think you enjoy more and whether you'd rather be in a big city or a small campus. End of the day, you need to go to both to decide.