Tungsten is right. Propane gives you like 50% of the power you should have. Besides that, I don't know why you would want propane. It's not like super cheap or anything, and you have to set it up.
All I am saying is that 2 of my wheeling buddies have propane kits on their rigs. One is a Toyota with the 22RE, another is a Samurai with the 1.6 Geo motor, both are making TONS of power, and Nismo is right... Propane is cheaper by far. The Toy runs almost a month on one cylinder, DAILY driven...