The ZDP-189 is harder than VG-10, so it holds its edge longer. VG-10 is more rust resistant than ZDP.Out of curiosity, how much nicer is the ZDP over VG-10 that's on the regular dragonfly? I always considered VG-10 a very nice steel for the money though I'm fairly new to the more premium steels. It seems like most recommendations I've seen recently on the dragonfly always add in the ZDP option as a "must".
If you truly want tough steel, then the H-1 used in the Salt line is the toughest they sell on small folders. In engineering terms, toughness is a measure of the work needed to fracture a sample of the material. Hardness correlates with/is a measurement of strength. Mild (soft) steel is tougher than hardened cutlery steels, but is too weak hold an edge. Generally, as hardness goes up, toughness goes down. Both properties are needed in balance with one another.