Originally Posted by Mapekz

Both Nasus's and Sion's ultimates lifesteal. Nasus lifesteals passively from all enemy champions around him, becomes immune to knock ups IIRC, and just hits harder and faster. He is a lifestealing off-tank that does an okay amount of AOE damage but is really good at chasing down and beating up single heroes.

Sion is more like a mix of DotA's Abaddon and King Leoric. He lifesteals with his ultimate, he has a shield that can explode for high damage and also takes damage for him, he has a single target stun nuke, and he has a toggleable that lets him drain some of his HP to hit enemies harder and, if he kills them when this is ability is on, gains a bonus to HP permanently. Many Sions get caster items to make their shield and storm bolt hit harder and to possibly instagib many heroes. AP caster items also increase the damage the shield can take, so combining this information with the toggleable and you can have very high effective health while still taking out people in 1v1 to 1v3 situations no problem.

Originally Posted by soulstealer

as far as damage goes, Sion. but if you want a champ that can assist well and help the whole team in teamfights i would personally say Nasus. imo the only thing sion brings to the party is his stun.