Hi, I was wondering the same thing.
So I called and they told me it is the same calling or on line.
Only thing is...calling will be faster because they can tell you which day and time the restaurant you want is available. For example, when I was looking for Be Our Guest ADR, I didn't have time to input every hour of every day to check the opening. But when I called, they could be able to find me the first opening in my 9 days stay at WDW.
Also, they said on line ADR updates every time there is cancellation (every second?) so I guess checking often will be the best way...(I became obsessed at some point...)
I would imagine people cancel a day before so don't lose hope. Maybe you can still find it when you get to WDW. Just call and ask!! Good luck!