My vote is Garden Grill over CM, but I'd do BOTH. At GG you don't have to watch your table to see if you need to run from the buffet to catch a character. Our server at CM told us to keep an eye out for characters and rush back if we saw one headed to our table because it takes an hour for them to make the rounds. I've been to GG 3 times, and every one we've had tons of great character interaction. It's relaxing having the food served family style. Mickey had plenty of time at our table and really played with the kids even though we were there at the peak of lunch. Every character was by multiple times and spent time with us. Dale came to join in singing Happy B-day. On a previous trip we had a late lunch, and we have video of Mickey and other characters dancing in the aisles with our kids. We had a constant characters at our table for 45 min. If you go off season and have late lunch you might have the characters almost to yourselves at GG, but I doubt there's ever a time you'll find that at CM.
Chef Mickey's was a great way to end our vacation because we'd already seen the characters at other meals. Had this been our 1st or only character meal, we would've been very disappointed with the interaction. We were there 2 days after Thanksgiving, and Mickey was making the rounds. Goofy was the one standing out front. We also went up to the 6th floor and looked down at CM on 11/22, and Mickey was making the rounds. I think Donald was out front. We forgot our camera when we went to CM, and half-way through our meal my dh ran out to get it. The characters gave our kids very quick hugs and pats on the heads when there were no autographs to sign or photos to be taken. When the camera arrived they stopped a little longer, but they were very rushed. We will go to CM next time because it's a fun atmosphere and the food was good, but we will make it late in the trip after we've had other character meals. Even the princesses at CRT were not as rushed at the characters at CM (except for Goofy out front).