Nitrogen is an inert gas and the molecules are larger than oxygen so maybe less leakage...
If you are airing up and down when off roading I wouldn't worry too much about putting Nitrogen in your tires.
Filling automotive and aircraft
tires[1] due to its inertness and lack of moisture or oxidative qualities, as opposed to air, though this is not necessary for consumer automobiles.
[2]
Nitrogen molecules are less likely to escape from the inside of a tire compared to the traditional air mixture used.
Air consists mostly of nitrogen and oxygen. Nitrogen molecules have a larger effective diameter than oxygen molecules and therefore diffuse through porous substances more slowly.
[3]
http://en.wikipedia.org/wiki/Nitrogen