It depends on how you like to tour the parks.
If you arrive the parking garage at 08:00am, get to the gate first, and go commando style, you can take a lap of walk on rides in the morning. This works any day of the year. Then you leave, go back to your hotel room, rest, and come back at night.
If you sleep in and show up at 11am, then it's another story. That's when everyone is there. 11am-4pm is rush hour at the parks.
If you want to be in the park during the afternoon, then it would be a good idea to stay onsite.
Plus the parks stay open late Christmas week, so you could come back at night, use your magic keys and never wait.
If you can save up the money for onsite and think of it as a splurge, then do it. Otherwise, you stay across the street, be happy you are saving money, and show up early each morning.