For my Flygon.
Iron Tail. It has lower acc, but neither of them are perfect. Think about it this way. for iron tail, 100 power plus 75 acc, plus 15 pp = 200 total. Steel wing 70 power plus 90 acc, plus 25 pp = 195.
Iron tail if you want power, wing if you want reliability.
I hate iron tail, but I will have to say it is better. Steel wing does not have 100 on accuraacy or power. I think at some level you need to have 100 on 1 of them.