My .02:
Consider the wind too. Maybe you are fortunate and don't have much, but around here a good windstorm can last a couple of days with gusts 50+ MPH.
After that, a cover will have flapped itself to shreds and/or sanded the paint right off the tractor where it rubbed.
A canopy is a better choice but must be anchored well so it can't fly off in the wind and cause injury or property damage.