Well, this depends on whether you want a sit-down dinner, or a cafeteria-style one. Cafe Orleans is a table service restaurant, so you have a waiter, etc. Plaza Inn you go through like a cafeteria with a tray, choose what you want, pay at the register, then go sit down. Totally different, and because there is no server, Plaza Inn is likely less expensive. That said, I've eaten at both. The chicken dinner at PI is HUGE and could easily be shared. You get 3 pieces of chicken, potatoes, veggies, a roll... BLEH I felt sick after and if I hadn't been solo, I should have shared! I like CO because of the yummy desserts and yes, they have the Monte Cristo also, which I share when I go because it's so rich.
If you want CO at a certain time though, I'd suggest calling for a reservation, especially if you're going at a busy time. I've eaten there without one though, just had to wait 20 minutes or so. Obviously Plaza Inn doesn't take reservations.