I have been to both, and both are wonderful and very different. But I would glady visit either!
Disneyland Paris Pros:
We can get there by Eurostar (which is my favourite method of transport) in under 2 hours.
The castle is the most beautiful of the Disney Castles and has a dragon.
Pirates of the Carribean and Buzz Lightyear are MUCH better
The Disneyland Hotel cannot be beat for location by any WDW hotel.
Everything is walking distance, and you never need to rely on busses/cars.
If you go at the right time of year you can have the entire park to yourself during EMH
Pro's of WDW:
The weather
The service/CMs are SO much friendlier and make you feel very special
There are 4 parks and water parks instead of just 2 theme parks
Much wider choice of hotels, and much nicer 'deluxe' resorts.
Tower of Terror and Haunted mansion are far superior.
Splash Mountain, Dinosaur, Liberty Square, Kilimanjaro Safaris and most importantly Mickey's Philarmagic are there.
In short. I love both. (And Anaheim too but I've only spent a day there so am not an expert)
If I could only have a 4 day trip I'd go to DLRP.
If it was 10 days or more I'd choose WDW.
A day at either would still be one of the best days of my year!