I can't tell you which to choose because they are both great. My kids, 7 and 3, want to go to both every year.
Chef Mickey's is buffet with tons of choices and great characters, and lots of fun and dancing. O'Hana is family style with great food, awesome juice and Lilo, Stitch and Mickey, and also lots of fun.
Food is great at both, though different from each other, so you may have to make your choice based on the characters your kids want to spend time with, and whether you prefer buffet or family style better.