Personally I would probably would go with the Nitride model. It is really a stainless GM/Knight barreled action with the Nitride hardening and finish applied to it.
Nitride finishes surface harden parts, but it is very brittle. Phosphate is a conversion coating for corrosion resistance. nitride is a pretty good rust preventative too, it'll also seem slicker just because it doesn't alter the surface finish.