If those are the only two choices, I would choose Chef Mickey's. It's bright and cheerful, the Fab Five will be there and the monorail goes through CR, which is pretty cool! The food, however, is atrocious. TRex is very loud and my DS was freaked out by the dinosaurs the first time we went when he was 4. I think it could scare a two year old. Chef Mickey's is definitely more festive than TRex.
If you want a character buffet with better food, I would recommend Tusker House. We celebrated my DS's 1st birthday at O'hana and had a blast with the coconut races and limbo, plus they bring everyone up who is celelbrating a birthday and sing a special song. If you really want characters, I would also recommend Garden Grill. I find buffets to be very difficult with young children. At Garden Grill, they serve you at the table family style which was always a more relaxing way to eat and meet the characters.