I have done both and when you figure in the cost of parking at the park, (if you don't want to wait for the shuttles from off site hotels- Holiday inn for instance only runs shuttles 4 times a day or so), it saves you more especially if you get a room only code from Mousesavers for a Value Resort- which the kids like better. None are available curently, buit maybe after Christmas? Yes the room is smaller, but how long will you really be in the room? It is the size of a Motel 6 or Comfort Inn. Even with naps- one parent will be swimming with the older one while the younger one sleeps with one parent watching TV or reading anyway if you are like us.
Some people maintain you don't even have to rent a car when staying onsite (we do and will next trip because we are driving from Tampa- since the flights from KC are cheaper there) but if you do and use it to go to the parks parking is free for resort guests.
The food is expensive there is the only drawback, but I have an answer for that too, I am getting Cereal and Milk and ice for the cooler at
Walmart (some have bought a dorm refrigerator) and we are doing breakfast in the room. Because I have anti trust issues with Walmart, I may try Publix when we are there. I am a hypocrite because I find myself at the big corporate monster because it is a half a mile away from my house, but I digress!