Santa's meant to have a physical presence here on Earth, which can effectively be disproved. Plus, his "actions" can all be attributed directly to other causes (with no close examination of causality).
The god thing has far too many loopholes to be refuted so easily. Believers can always say that it's because god isn't in our universe, but merely pulling strings from outside of it, or that god doesn't want to get directly involved, etc. and works "through" people, blah, blah, blah... There's always a new dodge, and god's actions aren't considered predictable.
Santa is really meant to go flying around the world each year, in a physically impossible way, and deliver presents that have never been seen, once you take out all the presents from parents.
I think you can fairly distinguish different supernatural stories from each other, based on what claims are being made. Like leprechauns can be disproved if the assertion is that you can find them at the end of *any* rainbow, and you create a small rainbow in a lab, where both ends can be seen at once.
