Generally, nitrogen specs are delivery pressure, oxygen content and water content.
For very large requirements of nitrogen, the most economical method is to use cryogenic air separation, where the air components (nitrogen and oxygen) are split in a distillation column. This is also used for obtaining liquid nitrogen, liquid or gaseous oxygen and also for getting argon.
For most applications PSA or membranes are used. PSA preferentially absorb nitrogen and the oxygen enriched air is purged. Membranes preferentially permeate nitrogen and prevent oxygen and water permeating through.
In most applications I have seen, membrane or PSA is the most useful. Selection is usually on cost alone. I believe that membranes are a lot more reliable as they have no moving parts (PSA have changeover valves), but I have yet to see any hard facts on this. Vendor's propaganda will not give you an answer, they will tell you the best solution is what they sell.