I've been to both twice each, but that was over 20 years ago. Both have interesting things to see. If you don't speak French, you can still get by, but London might be a bit easier. I can speak a little French snd I loved Paris. I think I prefer London, though. I suppose it depends on your interests.
IF you don't speak French PERFECTLY, the typical Parisian will correct your pronunciation instead of answering your question which you TRIED to ask in their language ! They will also pretend to not understand OR speak English.
Go to LONDON !
A spoonful of silly makes reality go away ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Poor reading comprehension by others is not my responsibility.