I think both are equally good as far as food quality, but they have several differences. 'Ohana is all you can eat served at your table. Chef Mickey's is a buffet where you serve yourself, so there is a greater variety to choose from. The atmosphere at Chef Mickey's is louder and more wild than 'Ohana, which is a little bit more subdued. The characters at 'Ohana are dressed in island wear; at Chef Mickey's they are dressed in chef's outfits.
If I had to choose just one, it would probably be Chef Mickey's just because it seems more "Disney" to me, but it would be very very close. You will enjoy whichever you pick.