We've been to both several times, and have a very strong preference for Garden Grill - it's our arrival night tradition every time!!
We think Garden Grill is way more low key than Chef Mickey's, the food is really good at Garden Grill (in our opinion!) and the character interaction at Garden Grill has always been some of the best at WDW! The characters seem to take more time with the kids there than other places, and if you sit on the bottom level in the booths, you don't see all the other people around you - just your family, the characters, and the scenes from the Land ride that gradually pass as the restaurant rotates. The other thing we love about GG is that the food is served family style, instead of buffet. So we don't have to take turns with our three kids getting food and juggling plates. We just sit, and the food and the characters come to us!
The only downside to GG for some (not for us, our boys love Chip and Dale) is that it's Mickey, Pluto and Chip and Dale. No Minnie. But again, our three boys don't mind that at all! And Mickey does have overalls to fit the theme instead of his traditional costume, in case that may matter to some.
Good luck with your decision, OP!