I have a Sig 226 .40 with the Nitron finish that I have carried as a duty gun for almost three years. Duty guns get pretty abused but the finish still looks great. It is not rubbing off anywhere even though it gets unholstered/holstered a lot. I think it is a great finish.
I think the Nitron finish was just developed. The toughest finish SIG had three years ago was their K-KOTE finish. It is advertised as being the toughest finish SIG Sauer offers.
Magnum,
Sig has another finish that is new and supposed to be tougher but the rumor is it is not allowed into the USA. Nitron is a finish over stainless slides. I think it is Sigs toughest blued colored finish.