The size difference in the two molecules is only 2.6 %. Consumer Reports did a test and after 1 year of sitting, the tires with nitrogen had leaked less....a whole 1.3psi less. But, since the "air" is already 78% nitrogen, the 2.6% advantage is only on that remaining 21.0% . The advantage (???) OK, there is some advantage, but it does not have much to do with the material, itself. If you waned a real "no leak" advantage, you would have to use argon. Thats too expensive.
By the time Nitrogen is put into a bottle, it contains very little moisture. For that reason you wont see as much pressure fluctuation as we do with compressed air. Most air compressors don't eliminate the moisture like bottled nitrogen.
If the place you are going to have a Nitrogen filter, forget it. The system a tire shop can afford is not worth the power it takes to run it...unless you have suckers willing to pay @ 8.00, per tire, to use it. Despite their advertisements, those systems really wont get much below 5% Oxygen so you would be paying the 8.00 to reduce Oxygen content from 12% to 5%....BFD,