Both are good, but personally I would go for Intimidate
Intimidate gives Salamence more switching opportunities, and forces out the opponent more as their attack is reduced, thus reducing the amount of damage they can do and potentially giving you a free turn when the switch. This gives you more room to set up your Dragon Dance. Also, by carrying Intimidate, even if Salamence gets crippled by say, Thunder Wave or Will-o-Wisp, he can still provide useful team support via his ability that Moxie variants would not be able to do.
This holds doubly true for Salamence in Doubles. Intimidate support in Doubles is much more useful and effective than Intimidate support in Singles, and should be the primary abilty over Moxie.
Intimidate is preferred, as it can potentially grant more opportunities to set up a Dragon Dance, as well as weaken opposing Pokemon. Moxie is an option if the extra Attack boosts are appealing; however, Salamence often cannot defeat its checks even at +1 or +2.
From analysis of DD set on Smogon X/Y