Here's some info that I *hope* will help you make your decision:
Chef Mickey's is buffet-style while Garden Grill brings food to you family-style.
Chef Mickey's is loud, frenetic and has a lot going on (music, napkin-waving, people going to the buffet, characters weaving in and out). Garden Grill is a bit quieter and more calm. Also, you should know that Garden Grill very slowly rotates as you're eating. We had no issue with this, but my mom felt seasick. (Of course, it could have been all in her head. Mom's prone to fits of "mom-ness" sometimes.)
The characters at Chef Mickey's are all wearing chef hats/aprons and at the Garden Grill they're dressed like they're on the farm.
At CM's you'll meet Mickey, Goofy, Donald, Daisy and Minnie...and I *think* Pluto (can't remember - someone jump in and verify, please!)
At GG you'll meet Chip & Dale, as well as Mickey...and I can't remember the others.
The nice thing about GG is that the tables are booth-style, so when they come around, it's a little easier to get pics without loads of strangers in the background. Not that this is terrible, but we've had a few interesting photo bombs.
Hope this helps!