We enjoyed both, but they are very different experiences so it depends what you're looking for.
For us, The Plaza makes for a better lunch spot. It's inexpensive and essentially a notch above QS. The place itself is very small and lacks atmosphere, but the food is very good. The food is primarily sandwiches, burgers, etc. IMO, better lunch fare. Chef Mickey's is obviously a buffet and character meal, so more expensive. Being a buffet, you have more dinner type entrees and can have a more substantial meal. It's also a loud, crazy, atmosphere where the kids seem to have a good time.