The atmosphere at both are great, but I was not at impressed with the food at Coral Reef. I did however love the food and everything about the Garden Grill.


. Now, as for Garden Grill we are dining there for the first time in December and are very excited about it. We are doing 6 character meals this trip and GG is the one that I am most excited about doing. First off the food looks amazing and I love the fact that the restaurant rotates as you dine...plus C&D will be there and my DD8 is a huge fan of those little rascals.
Reef. 
We have eaten at both Coral Reef and Garden Grill. The food at CR was not very good ,but the fish tank made for a cool atmosphere.Personally, I would choose the Coral Reef. I was really not impressed with the food at Garden Grill. The characters are great and does one really ever outgrow Chip & Dale (my favorite characters). But the steak at Coral Reef was better than my steak at LeCellier (and dh and dd both agreed) and I love the atmosphere of the restaurant. We were on the top tier but still had a clear view of the tank. We are visiting the Coral Reef again in 26 days.

We have not been to the GG, but of all the places I will never go back to @ WDW, is Coral Reef. The atmosphere is cool, but the food is at best mediocre, and its kind of dirty.
There are at least 6 or 8 better choices in Epcot alone IMHO.
Including:
Le Cellier
Chef DeFrance
Nine Dragons
Tutto Italia
AND THE HOT DOGS @ American Adventure!
Have fun!