UCL might be a technically better University, however it does have its shortcomings. It has been very unforgiving and very uncaring to some of my friends who went there. Sussex is a very friendly place in my experience and is world renowned aswell, particularly for international relations (do a Google search for "International Relations" and see where Sussex are in the results).
It does depend on what you want to do with your life too. I realise its a bit early for you to know really but if you do want to go into International Relations type stuff then you'd need to have a masters in it if you don't have an undergraduate degree in it. While you'd be very well qualified with an undergraduate degree in Russian Studies and a masters in International Relations, a masters degree costs a good chunk of money. Living in London for three years + the cost of a masters degree = a lot of debt.
I couldn't say which would be a better choice for you. My guess would be that if you don't quite know what you want to do with your life then go for UCL. If you know that International Relations is what you want then have a good long think about your options, funding, worries of debt. I would hasten to add that although debt is quite a scary thing, if you can get a good job at the end of it then its kind of worth it. Good luck with your decision!