Both.
Mimi's had a good veggie burger and were helpful with my son, who is vegetarian (we all are) and has issues with certain ingredients. But cheap, we did NOT find it to be. Of course if we'd gone with our instincts and shared the veggie burger between hubby and me, it would have been better.
Millie's, however, was so cheap for what we had! We had take-out one late night when DS wasn't hungry until JUST after restaurants started closing at DL and we were leaving the parks anyway. One order of mac and cheese, one salad, one order of garlic bread = something like 11 with tip? Was still hot when we got back to hojo, filled up a ravenous 4 year old and two "eh, I could eat" adults. DEEEEELICIOUS, as well.
So Millie's has my "what we had was better and cheaper" vote, but since we haven't had the same thing at both places, I can't really compare, so I say BOTH!