A lot of people forget to factor in Stone Edge's 20% chance to miss. You can either choose one of the 100% accuracy moves to do lower yet consistent damage, or choose a stronger move that has a 1 in 5 chance of not doing any damage at all.
VS Specially Defensive/Paraflinch/Defensive Nasty Plot Togekiss:
252 Atk Garchomp Iron Head vs. 248 HP / 0 Def Togekiss: 182-216 (48.79 - 57.9%) -- 56.64% chance to 2HKO
252 Atk Garchomp Stone Edge vs. 248 HP / 0 Def Togekiss: 228-270 (61.12 - 72.38%) -- guaranteed 2HKO
+2 252 Atk Garchomp Iron Head vs. 248 HP / 0 Def Togekiss: 364-430 (97.58 - 115.28%) -- 87.5% chance to OHKO
+2 252 Atk Garchomp Stone Edge vs. 248 HP / 0 Def Togekiss: 454-536 (121.71 - 143.69%) -- guaranteed OHKO
VS Offensive Nasty Plot Togekiss:
252 Atk Garchomp Iron Head vs. 176 HP / 0 Def Togekiss: 182-216 (51.26 - 60.84%) -- guaranteed 2HKO
252 Atk Garchomp Stone Edge vs. 176 HP / 0 Def Togekiss: 228-270 (64.22 - 76.05%) -- guaranteed 2HKO
VS Physically Defensive Togekiss:
252 Atk Garchomp Iron Head vs. 248 HP / 216+ Def Togekiss: 136-160 (36.46 - 42.89%) -- 97.56% chance to 3HKO
252 Atk Garchomp Stone Edge vs. 248 HP / 216+ Def Togekiss: 168-198 (45.04 - 53.08%) -- 0.39% chance to 2HKO
+2 252 Atk Garchomp Iron Head vs. 248 HP / 216+ Def Togekiss: 268-316 (71.84 - 84.71%) -- guaranteed 2HKO
+2 252 Atk Garchomp Stone Edge vs. 248 HP / 216+ Def Togekiss: 334-394 (89.54 - 105.63%) -- 37.5% chance to OHKO
So in retrospect I'd say it's actually about even when it comes to Togekiss. Stone Edge's annoying 20% miss rate is partly offset by the increased critical hit chance scoring some OHKOs, such as against Nasty Plot variants (which are the sets most likely to threaten Garchomp).
That being said, Iron Head's 30% flinch rate can turn any 2HKO into an OHKO in a pinch, still gets a boost from Sand Force if you go Mega, and covers all Fairies bar Azumarill when combined with Earthquake's coverage.
I guess it depends on which move type your team needs most. If you're already running an offensive Steel-type (such as Aegislash or M-Mawile), go with Stone Edge to cover threats like Moltres, Charizard and Rotom-Heat. If you're looking to run Garchomp as a sweeper, Iron Head looks like it does the better job of removing Fairy-types wanting to stop your sweep.