There is a huge difference in the two and it depends on what you are looking for. Downstairs, crowded, noisy, and bussling with life is the expensive and delicious Les Chefs de France. The meals are traditionally French (take it from someone who spends a month a year in Paris)...It is also the less expensive of the two (Although not cheap - expect entree's around 30 dollars each.) A meal here will run you about 50 dollars a person (not including wine). It is significantly more expensive for dinner and depending on what you order you could be paying significantly more. Check with your waiter to find out what is included and what is NOT included in the dining plan.
Upstairs, and very different in atmostphere is Bistro de Paris. It is quiet, beautiful, and much more formal. It is one of the few restaurants that I tend to dress up for at WDW. Entree's here are in the 40 dollar range. Wine bottles start at around 50 dollars and can go up significantly depending on what you want. Expect to pay about 90 dollars a person here for dinner (not including wine), and significantly more depending on what all else you order. Bistro de Paris IS NOT included in your dining plan. Try to ask for a window seat here and you have a fabulous view for Illuminations. This is by far the more upscale of the two restaurants, but it is more romantic.