Personally, I'm glad I did the experience of watching them from the top deck of CG once, but in my own opinion I like watching them better from Main St. in MK. I just really like being right there in the park and seeing them from there better. Plus, while pretty much any area on Main St. gets you a good view, to get a good spot on the outdoor deck at CG you need to go out at least 15 mins early, and although they let you return to your table when done, its kind of annoying to have to get up for 1/2 hour to go out for them. You can see the fireworks inside too, but the view isn't as good unless you have a great window seat.
Personally, I like to go to CG earlier for dinner when it is daylight out, because I love that sunset view over the Magic Kingdom and the castle. Then, go see the fireworks from inside the park later on. It is just a short ride over by monorail or nice walk. So in summary, we go to CG for the daylight view and great food, not for the fireworks.