No personal experience storing an in-mast main over winter, but I'd probably opt to take it down. At the very least, that allows for a closer end-of-season inspection of the sail and furling gear so there aren't any surprises in the spring. That would also protect the sail from any water/ice that may find its way into the mast track/sail and potentially cause problems.
How hard is it to take the main off?
How hard is it to take the main off?