I see mostly AP Sion's these days, but I think a Sion AD Off-Tank is a viable option for Sion.

Enrage gives Sion so much flat damage that when playing Sion as an Off-tank you really don't need to look for much attack. For this reason I max out Enrage as soon as possible because it gives me the ability to farm minions well while only last hitting, and then from that farm I can then get the HP from Enrage as well. I get the Phage after boots because Sion is somewhat soft early on partially because the ability Enrage takes a toll on his health, but after buying this I am able to buy a BF Sword to deal some good damage mid-game. As the game speeds along though Sion with his ult is best for his entire team when he can stay alive that is why I build like a tank as the takes its course.