The milspec finishes for guns are tough, I think we can establish that, so given that here some info I read somewhere some time ago...
Duracoat lasted around 300 hours in salt spray tests, which is FAR past mil spec.
KG Gun Kote was something around 500 hours
Ceracoat C was 350 hours and the H was 1000 hours.
There are coatings like PE and PVD that can offer 1000+ hours of ACID SALT mixture spraying. I know this because I am making some knives and considering what coating to use.
I'd say, that Duracoat is the best bang for the buck, although some people wouldn't consider it cheap. I believe Ceracoat H is about double the price and PVD (DLC), is even more. So, at some point there is delimiting return on the investment of a coating. Done properly, duracoat will last a long time...