Paris is more of a tourist trap than London, and have been more so every single time I visit (3~4yr gap each time), though I am sure you will be able to avoid that if you live there. Also you speak French fluently, so probably gets a very different experience than I do. I do like Paris on each visit, but would not live there until I can at least speak French. I can certainly imagine study there, but probably not work there. London is multi-culture, probably the most multi culture city in Europe, but not in a multi-culture American way. America is still an immigrant country. Maybe it's because I have only been here for 1.5 years (11years in US), but the impression I got is people go to London to work for a few years then go back to whatever they're from. There seems to be a higher percentage of people who actually decides to stay in America for good. Also I am still somewhat disappointed by food offering in London, yes there are good ones, but you got to pay. There are a few reliable cheap local charm, but they're far and few. The ethnic food (for a lack of better word) in London is all over the shop, and some definitely changed quite a bit for local taste. The multi culture aspect doesn't really stand out as much as America (or maybe they're just better integrated here than America, racial tension is really an interesting case here, but that would be completely off topic). All in all, never lived in Paris, so can't say how it is to actually live there. You can have a lot of fun in London (clubbing/cultural/art/interesting experience) and Paris is always one train away. Personally if I was to just live somewhere in Europe for fun for a year I would definitely go to Southern Italy..., not very helpful conclusion I suppose
