Yikes! That is a tough question because those are my two favorite Signature restaurants. I've been to Narcoossee's four times and California Grill once.
Narcoossee's has amazing food and service. I've has the whole Maine lobster several times and they've all been cooked perfectly. The calamari and crab cakes are also excellent. For dessert, the chocolate mousse is my favorite, but the cheesecake is also really good. Our servers have also been very knowledgeable and and tentative.
At California Grill, the food and service were also really good. The sushi was very good as was the crab stack. The Filet was excellent, as was the snapper. The mousse was yummy.
Both Narcoossee's and CA Grill have great views of the fireworks. You might get lucky and get a table with a view, but you can go outside to watch. Narcoossee's has a veranda (anyone can watch from there), while CA Grill has the observations deck (only guests of the restaurant are allowed). The show's music is piped in at both locations.
The view of the electrical water pageant is better from Narcoossee's. It stops right in front of the restaurant and does it's show.
Overall, if I were only allowed to eat at one place I would pick Narcoossee's because I really liked the food.