Autumn Rain

Iron tail is usually not worth it. Iron Head is a reliable move.

Wait until it evolves into an Aggron and gets Heavy Slam. That's almost always a much more powerful move simply because Aggron is so heavy. Better still if it has Heavy Metal as an ability.

Rather than generalizing it, it's dependent on the Pokemon that has the choice between these moves. If Iron Tail's stronger power is important to say, 2HKO a Fairy while Iron Head cannot, I'd find Iron Tail superior. But if Iron Head is enough to 2HKO something, well, it's easily better than Iron Tail, but doesn't apply to everything.

Iron tail has a chance to drop defense which could be useful against some bosses...