if you opt for DTD you can get something similar to an ADR by calling the restaurant direct. 407-827-8500
As mentioned by the PP there's not that much difference. I think the choice is driven more by your schedule than anything. For example we often arrive at Disney World late afternoon or early evening. Downtown Disney is a place to get in the spirit and not waste a park admission when we would be tired and have only a couple of hours. We head to DTD for a meal and some shopping and get to bed early to start the touring refreshed in the morning.
If you're looking for somewhere to eat on an AK day then RFC may work there.