We ate at Coral Reef in January 2005 (when the dining package first came out, and it was 2 meals on the meal plan). It wasn't worth 2, but it's worth one. The food was decent, nothing incredibly special but it was better than some of the other meals I've had at the world. The environment was WONDERFUL with a personal view of the tank (we sat right next to it and watched the sea turtle and a GIGANTIC grouper through the whole meal). However, as other people have mentioned you can get the same view (or better) from inside the Seas itself - where you can also see the dolphins, manatees, etc.
We ate at Chefs de France on our trip last week for the first time. We were hesitant to try it, but it ended up being my favorite meal of the week. Our server was actually originally from Italy, but had lived in Italy, France, Portugal, and a few other countries and used to work on a cruise ship - he was wonderfully friendly and helped us choose what we'd like best. Our meal was delightfully tasty (and there were 6 of us at the table so we all got a different dessert and shared - they were DECADENT). The atmosphere wasn't as interesting as Coral Reef (it's representative of a cafe bistro in France - so it was just round metal tables & chairs near each other)...but we sat right near the window so we could see outside onto the lagoon, and we finished just before Illuminations so we had a great viewing location.
So if you're looking for the better food, I'd go with Chefs de France. If you're looking for better atmosphere (particularly if you know you enjoy aquariums/the Seas), go with Coral Reef. If you're torn between the two, I'd pick Chefs de France.