I would do Garden Grill if you have already done Chef Mickey's before. Garden Grill is fun and different. We enjoyed the food there. Chef Mickey's is hit or miss with food...usually miss. LOL
GG-food is much better (IMHO). True, there are more choices on CM's buffet but I find the quality much better at GG. Characters tend to stay a little longer and hang out with the kids at GG. Always feel like CMs is rushed not to mention over-priced and over-rated.
I think it depends on whether you want to go for breakfast or dinner. We've been to both and liked both. CM for breakfast was, we thought, really good. We hit it a little later and thought it worked fine.
However we did lunch at GG (don't think they do charecters @ lunch any longer) and loved it! The charecters were great and either chip or dale (can't remember which) "ate" DD's mac and cheese. That was 3 years ago and she's still talking about it.
DD has never done CM, DH and I did that by ourselves back in Jan. and had a ball! The charecters gave us just as much attention as they did the little ones