Hmm. I typically go with the most accurate move, since accuracy tends to fail me when I need it most (GLARES at the TM for stone edge) but that's just my luck. If you're the kind of person that hits a lot with hydro pump, blizzard, and stone edge, you'll probably get the same luck with iron tail. Both moves have the same amount of power points (PP). Iron head has a 30% chance of making the target flinch, while Iron tail has a 30% chance of lowering the opponent's defense by one stage.
I'd go for Iron head for reliability and decent power.