I've been to both recently and London was better, Paris is very overrated and I had expected much more. The Eiffel Tower was really amazing, just like the sightseeings in general, but the rest was so... bleh. London, on the other hand, was more cool and enjoyable (maybe because I didn't expect a lot, UK is generally one of the least interesting countries in Europe for me).
Both of them were dirty and that's what I disliked the most.