Flash Cannon is indisputably better than Steel Wing. Steel Wing is not a move you should be using in a competitive setting. It is too weak and will not secure the OHKO Hydreigon needs to win against Fairy-types.
If you want to go physical, use Iron Tail instead, as this move is at least workable in terms of its base power. Keep in mind however that this is still very much a niche alternative to Flash Cannon, as it's less consistent, less reliable and forces you to run Naive. Unless you're looking to deal with the likes Sylveon and Togekiss in particlar, go purely special.
Flash Cannon is the best general answer to Fairy-types that Hydreigon has, though keep in mind that Fairy-types still have the upper hand over you even if you use this move. Steel coverage on this Pokemon is best used when you predict the opponent will switch to a slower Fairy-type, so you can pressure the opponent or 2HKO their switch if they do follow through with it.
