A nitride finish will give the longest lasting protection of the two. Nitride finishes are actually part of the firearm while DuraCote/CeraCoat etc are just painted on. The spray coatings give you a better choice of colors and patterns while nitrides are either gray or black. Nitrides are slicker than the coatings which may help functioning. Coatings can be easily repaired or even changed if the user wants.
One of my favorite finishes is Black T.