It depends a lot on what you want out of your dining experience. We really look for ambience, so both these restaurants were very enjoyable for us. Depending a lot on the server, there can be a lot of interaction at Prime Time, so don't go if you want to just be left alone. Otherwise, the food is good, it's a very retro kitchen atmosphere, very fun and very "Disney" unique.
Coral Reef's food was good, but I really like the aquarium atmosphere. It's like a 5 million gallon tank or something, and very soothing, lots of blues and greens, and dark, so it's a nice place to take a break and get out of the heat. Our CM told us that they their dessert has been voted "Best in WDW" for a couple of years now (can't confirm, but that's what she said). Either way, it WAS very good, called the "chocolate wave". I loved it, and I'm not even a chocolate person.
If you're going for something new, you should try one of these at least once, especially if you're already in the parks. But the resorts also have great restaurants, you almost can't go wrong!