Parkerizing is about the cheapest finish youre going to encounter from the standpoint of having it done commercially. It is easily done yourself if your handy and have access to a bead blaster and SS pot.
It is only barely a step above bluing as far as rust resistance though. A lot of military arms are parked and then painted, which takes care of the corrosion problem nicely.
Park on its own wont hold rust at bay long if you dont constantly stay on top of it, especially in harsh conditions.
Paint is very effective at resisting rust, but I havent seen it hold up to well on a handgun that gets used on a regular basis.
If youre planning on carrying the gun, and are worried about rust, then something like hard chrome, or one of the other more current finishes are best, but they are also more expensive, and not really "GI" looking.