I haven't been to the Planet Hollywood in Disney, but have been to others around the country. We have an ADR for Coral Reef for our trip in January. Obviously it's a personal choice, but there are few things working for both.
There is only 1 Coral Reef plus the views of the aquarium; there are Planet Hollywoods in many cities (and it's basically burgers or entree salads for me)
BUT Planet Hollywood would have a more varied menu (depending on what you want to eat) and is most likely less expensive
And the fact that you'll already be in downtown Disney.
Personally, it would come down to location, what you feel like eating (who else is in the party and their appetite), your budget, and also if you have already enjoyed a number of other Disney restaurants; if so, you can always save Coral Reef for your next trip.