It only scales with item health unfortunately, so Sion's W passive doesn't work with it. It only gives him as much health as any other tank would get from the item. This makes Sion one of the worst users of the item since he already gets tons of HP from his passive, so you should look for items that scale with HP or items with resistances instead of more HP.
The tooltip in the shop will lie about Heartsteel scaling with max HP, even lying about the Heartsteel's numbers increasing when you get W stacks. But if you actually test it you'll see you won't get more HP or damage from Heartsteel no matter how many W stacks you have.

Go with the tank build by default, and go AD against champions with true damage or armor pen, like Gwen, Olaf and Trundle. Another important consideration is whether your team comp will benefit more from a tank or an AD assassin. If the enemy team has more immobile squishy champions, this is favourable to AD Sion. If you're new to Sion, go tank Sion.

I wouldn't recommend Triumph mainly because Sion can't afford to use the precision tree generally. Thanks for your friendly comment