I think all the other posters have given great input, although at the end of the day it's all going to depend on your preferences.
Disneyland Paris is the most beautiful castle park there is, and it has the best Space Mountain, Big Thunder and OG Pirates. And by the time you visit there'll be Avengers Campus. I try to visit 2-3 times a year, and always for 2-3 days, but I also like to take DLP slowly and just enjoy the atmosphere, rather than running from ride to ride.
May in Nice should be beautiful. A note that I don't think anyone else has mentioned, you really need to consider a car rental, if you haven't already planned to. You don't need a vehicle in London or Paris, but most of the rest of France is hard to get around without one. I'm assuming you'll be travelling from Paris to Nice by plane? The entire coastal area in southern France and Northern Italy is wonderful. I could list a dozen outstanding road trips to do in France, but that coastal drive from St Tropez to Cinque-Terre is to die for. Plus you get an Italy stamp in your passport en-route.
Personally, if you can, I'd try to visit both. Which, I appreciate is 100% not the advice you were looking for!!