I was actually torn between the two as well, because I wanted a signature experience for my last vacation night since we didn't want to waste our last day at a TS restaurant. Consider the type of restaurant you are going to and the atmosphere and what is best for your party.
I made somewhat of a list of the pros and cons of each for my party.
Atmosphere: I like that at CG you are in the top of the Contemporary and have a nice view of the fireworks and the surrounding area, but I prefer the nice waterside area of Narcoossee's, and it is also a nice plus for me that although I can see Wishes, I can also see the water pageant, I made sure that I booked a time when I could catch both of them.
Food: I've heard a lot of great things about the food at CG, including the sushi, which is something I only sort-of like and I'm sure that my dining companion will never even consider trying. The steaks are nice, I suppose, but we are already dining at a steakhouse on our trip. Besides, my friend and I both LOVE seafood and she is looking forward to the lobster (me, I HATE lobster! But love other seafood <3)
But generally when it came down to it, being from New England, we prefferred the waterside seafood restaurant to the more contemporary (hehe) restaurant that seemed to be more of a steakhouse. The view and food both fit what we wanted, never having been to either of them before.
So..what I am saying is take into account the things that the group you are dining with will enjoy. I asked DBF which he would prefer if he was going and he said CG...for the exact same reasons as why I picked Narcoossee's, but the reasons pertaining to CG. So it really is up to the food and atmosphere preferences you have.