I personnally prefer the food at Goofy's Kitchen, primarily because of its variety. I think the food there is a little better than average for a buffet, and my son always finds plenty to enjoy. I found the food at Ariel's pretty boring. The menu choices are pretty lacking, especially for those that like to eat healthy. My son, who loves meatloaf, found Ariels bland; I had the pasta that was also boring. Others like the food though.
The wait to get into Goofy's can be a bit long at times, even with a PS. I've had times when I was seated right away, and other times when the wait was close to 1/2 hour. I've only been to Ariel's once and believe I had to wait about 15 minutes for my seating there, but generally it's not as crowded as GK.
I always love the characters at Goofy's. There's a great variety. Goofy is always a hoot, and there's generally at least one "face" character and/or princess. But if your daughters love their princesses, Ariel's might be the better choice. The character interaction is the primary reason to go to these restaurants.