TargetTalk

I would summarize this way. If you have easy access to Nitrogen (easier than compressed breathing air) use it. It would be extremely safe and probably would cause less deterioration to cylinders, although we can all agree than they cylinders will outlast their stamped service life regardless of the gases used.

There is risk in the event a cylinder fails with Nitrogen regarding insurance policies. The risk of this (cylinder failure causing bodily injury) is extremely small. You're far more likely to be hurt in an auto accident today. But there's even a chance that some fine print somewhere will also deny your claim if the cylinder fails with compressed air just as it would for injury during sky diving or racing a car.

Nice summation by OZ, with one comment: if the thing fails most probably it is your fault, either by careless handling (strap the thing to the wall) or lack of maintenance (15 yrs old and going strong, but did you ever look at the inside?).