Hi Patricia,
You can't really go wrong with either one of them.
Chef Mickey's is incredibly popular because the atmosphere is fun and the buffet delicious. You'll often see Mickey, Minnie, Goofy, Pluto and Donald.
The Good Morning Gathering is only offered to Guest's of a Magical Gathering (8 or more Guests over the age of 3), so you wouldn't be able to do this on future trips unless you were traveling with a large party again. For that reason, if I had to choose between the 2, I would pick the Magical Gathering.
I do have to say, that I also recommend that you book dinner at Chef Mickey's for a different night. The kids are all the perfect age to enjoy it!
Hope this helps and have a wonderful vacation,
Kaylene