Phosphate is a metal treatment to impart corrosion resistance. Still one of the better finishes on the market for someone making a "hard use" rifle.
Melonite is a a metal treatment to impart corrosion resistance which also has the added benefit of being hard and "slick" very similar to chrome, which will allow the bore to last longer than that of an untreated barrel.
If I were to choose between the two with no regard to cost I would choose melonite.
Jimro