We have eaten at both. Our experiences were quite different than others who posted here. In general, both of these are quite popular and you will likely enjoy either one. One person's "favorite" is always another's "never again". All it takes is one bad server or an over cooked steak and the meal is ruined.
With that said, it is unlikely we will ever return to Coral Reef. We, unforunately, had very poor service (food was cold, wrong items brought out, waited 10 minutes for one of the kids meals after everyone else was served, no refills, etc). None of us enjoyed our food. It wasn't so much bad as it was unmemorable. We were also seated along the back wall (not close to the fish) and waited over 30 mintues for our ADR.
'Ohana, on the other hand, is our favorite restauarant at WDW (we have never dined at a signature). We had EXCELLENT service, fresh and delicious food, a window table with beautiful views and really enjoyed the atmosphere. We LOVED the sticky wings, steak/shrimp and the bread budding was to die for!
I have to say, I recommend 'Ohana!