Propane is fuel, nitrous is air. You obviously need both to run your truck, but you need more fuel first, so I voted for propane. Another advantage of propane is that you can easily carry enough with you to avoid refilling very often. With either nitrous or water/meth, you're only using the stuff for short periods, and you're constantly refilling your tank.
If you're looking for a tank, contact Dale Isley at TYMAR. He sells Manchester tanks, and made my custom brackets for mounting mine under the truck here.