You can leave it cocked almost indefinitely. A metal spring will take a "set" and loose power. Some feel that the shot cycle is smoother since there are no vibrations like you get in a metal spring. Although others say the shot cycle is a bit sharper with a tad more recoil. On the downside, if a gas piston fails out of warranty, it will cost you a lot more than simply replacing a wound metal spring. But they hold up pretty well.
The Nitro Piston is proprietary to Crosman. I'm not sure they are making pistons for guns other than their own, but I could be wrong. Some guns, like the Diana springers, cannot currently be retrofitted with a gas spring due to the way the trigger mechanism works.
Rather than buy a gun and retrofit it, I would look to one of the Crosmans.