If not going with Sacred Sword and choosing a Steel move instead, it can be a tough decision between Flash Cannon and Iron Head. With maximum SpA investment, as is most common with Tank Ageislash, Flash Cannon does seem superior for general spamming. It allows you to very solidly dent Mandibuzz more than Iron Head (39.6 - 46.6% vs 20 - 24%), and actually outdamages Iron Head versus common Fairies and Rocks:
- 252 HP / 252 +Def Clefable: 88.8 - 105% for Flash Cannon, 54.8 - 64.4% for Iron Head
- 252 HP / 252 +Def Sylveon: 64.4 - 76.6% for Flash Cannon, 58.3 - 69% for Iron Head
- 0/0 Terrakion (non sand): 116 - 137% for Flash Cannon, 83.5 - 98.4% for Iron Head
- 0/0 Terrakion (sand): 72.4 - 85.4% for Flash Cannon, same Iron Head
- (Tyranitar is within a few percentage points of each other either way)
However, the main draw to Iron Head over Flash Cannon is the nastiness of Calm Mind Fairies. Clefable can come in on something, Calm Mind as Aegislash switches in, and Calm Mind again before Aegislash Flash Cannons if you've revealed you carry that instead of Iron Head, and sweep away. Calm Mind Clefable variants are surging in popularity, and Aegislash with Iron Head is one of its bests checks. If you carry the Steel move to stop all Fairies ever, Iron Head is the way to go. If you want the most spammy bang for your buck (or don't want another contact move, since Rocky Helmet is also surging...), Flash Cannon for you. I alternate between the two based on what threats seem most popular at the time. Right now is a Flash Cannon phase. XD
Hope I helped!