oooh tough call! We were there a few weeks ago, and it was our first time at BOTH GG and CM.
I had heard some negative reviews about CM, but we had a blast, and the food was great (at least for a buffet imo). Everyone loved the desert area, the kids had fun with the napkin twirling and the characters all came round in a timely manner.
However, we were stunned by the GG. The food was really really good (our best meal of the 10 days!) and the characters doted on us, coming around 2 or three times, and really had fun with the kids. It was really neat the way the restaurant revolved, and really peacful since you are basically very private and not facing other tables. The server was fantastic (DD broke her glass when she dropped the butter dish on it, and they had the table cleared, cleaned and re-set with new dishes, drinks and bread basket in literally about 2 minutes!). I would highly recommend this one!!