I highly recommend ordering in advance. Every time we've had lunch here there has been a long line across the bridge for those who did not order ahead of time and were waiting for a kiosk. Those who pre-ordered walked right into the castle to a very short line to pay and then find a seat. I only made the mistake once of not pre ordering and won't be doing that again.
Also, keep in mind you can preorder right up until just before you check in for your ADR.