As with all things, it really depends on your team. If you're still running D'Art/Mammon and using D'Art gun then the additional HP from medjed's gun has diminished value, while the higher assault on the axe has increased value since you are already dedicating another slot on the grid to HP.

Of course, if you're not running any other weapons that gives HP in your grid and you find that you'd want a bit more HP, then medjed's gun is a better value as a dual skill weapon.