I have *only* been to Disneyland Paris and only as a child/young teenager, so my comment will be biased. I'll be going to WDW in Orlando for the first time in ten days so I can better compare the two soon!
Disneyland Paris is very compact, doable in one or two days, and absolutely beautiful and charming. There is the main park with most of the Magic Kingdom attractions and a second one that seems to be a spin on Disney Hollywood Studios in Orlando (I haven't been to the second park so can't comment on it too much). Disneyland Paris has several unique attractions that I loved as a kid and that are different from anything in either WDW or the original Disneyland. Tomorrowland, for example, has a retro-futurism theme inspired by the work of French author Jules Verne, and the Phantom Manor is a dark and scary version of the Haunted Mansion with more of a Wild West theme.
Depending on the length of your trip, I would take a day or two and spend it in the theme park - I believe it's only an hour or so outside of Paris by RER train. However, if you only have a week or less, and you haven't been to Europe before, I would skip it and focus on more traditional experiences such as the Louvre, Eiffel Tower, exploring the many beautiful neighborhoods of Paris, etc. I was in Paris for a week last year and didn't take the time to go out to Disney because there was so much I wanted to do in the city proper. We just couldn't find the time in our schedules. There are many different ways to experience Disney parks, but there's only one Paris!