Iron head on either CB/DD misses out on quite a few 1HKOs an 2HKOs (forgot which ones) when iron tail will guarantee many. The accuracy does indeed suck but for the most part, I'd take on iron tail of iron head. I've actually found DDLumNite to REALLY love iron tail to cover its fairy weakness, which is its kryptonite of this generation...unless it has iron tail. It really misses on having earthquake or fire punch for the steel types though...
252+ Atk Choice Band Dragonite Iron Head vs. 4 HP / 0 Def Mega Altaria: 272-320 (93.1 - 109.5%)
252+ Atk Choice Band Dragonite Iron Tail vs. 4 HP / 0 Def Mega Altaria: 340-400 (116.4 - 136.9%)
252+ Atk Dragonite Iron Head vs. 252 HP / 252 Def Unaware Clefable: 190-224 (48.2 - 56.8%)
252+ Atk Choice Band Dragonite Iron Head vs. 252 HP / 252 Def Clefable: 282-334 (71.5 - 84.7%)
252+ Atk Choice Band Dragonite Iron Tail vs. 252 HP / 252 Def Clefable: 354-418 (89.8 - 106%)
252+ Atk Choice Band Dragonite Iron Head vs. 252 HP / 252+ Def Sylveon: 276-326 (70 - 82.7%)
252+ Atk Choice Band Dragonite Iron Tail vs. 252 HP / 252+ Def Sylveon: 346-408 (87.8 - 103.5%)
* Both moves OHKO Sp Def Sylveon.
252+ Atk Choice Band Dragonite Iron Head vs. 252 HP / 0 Def Tyranitar: 272-320 (67.3 - 79.2%)
252+ Atk Choice Band Dragonite Iron Tail vs. 252 HP / 0 Def Tyranitar: 340-400 (84.1 - 99%)
252+ Atk Choice Band Dragonite Iron Head vs. 0 HP / 0 Def Azumarill: 177-209 (51.9 - 61.2%)
252+ Atk Choice Band Dragonite Iron Tail vs. 0 HP / 0 Def Azumarill: 221-260 (64.8 - 76.2%)
252+ Atk Choice Band Dragonite Iron Head vs. 252 HP / 0 Def Azumarill: 177-209 (43.8 - 51.7%)
252+ Atk Choice Band Dragonite Iron Tail vs. 252 HP / 0 Def Azumarill: 221-260 (54.7 - 64.3%)
Both moves OHKO Diancie and Gardevoir.
So it seems both moves claim the same 2HKOs but Iron Tail gains some important OHKOs. Both attacks are viable but Iron Tail does claim more OHKOs without relying on prior damage or Stealth Rocks. Sucks if it missed though.