The difference between a dirt floor, gravel floor or concrete floor is how much moisture comes up through the ground to corrode your tractor. With any of the three, you would do better to raise it above the surounding ground. Then I'd say gravel is better than dirt because gravel drains better and doesn't trap moisture as close to your tractor. Finally, concrete is best because it actually is a barior. I've heard people say that concrete will wick water up - however, I've nover seen any evidence of this. For my barn i used a foot of clean gravel and 6" of concrete above that. The floor of the concrete is about 8 inches above the surounding ground. It is dry as a bone despite being in an area of the yard that is typically very wet.
I have kept my mower in the barn for years and never have had rust on the blads. After one night outside, rust on the blades.