Ad/as benefits your ulti, but for an over-all good aoe champ, ap is the way to go.

Many first time Sion users think Sion is a great tanky, dps champ is he does ad/as. This is not true. By late game this guide will give you around 2.6-2.8k(normal game) health plus around 800dmg for a shield. The main abilities used by ap Sion are "Q" and "W". Your "W" will 1 shot creep waves benefiting your "E", giving you that 2.6-2.8k health by end game. "Q" will do about 700-900dmg as well as your shield when getting most the items in this guide and over 1000dmg when maxing out all items.

For marks it is best to get magic pen so your abilities will do more dmg to tanks especially tank/supports like Galio. For seals I use mana per 5 to get past the high cost of Sion's abilities, for glyphs it is best to use straight AP as well as for quints

I start out with a a meki pendent so you can stay in your lane longer. After that I follow the item list above. If you are not doing so good I would recommend building the Needlessly Large Rod and skipping the Mejai's Soulstealer until later game.

Pros
- Mass damage out put
- More survivability
- Overall more fun

Cons
- Ulti does not do as much damage
- Slower attack speed and run speed

Most players will say "Flash" is better than "Ghost" for many AP casters this is true, but however Sion is not a caster. Due to being an AP build many items that would normally increase Sion's movement speed are no longer used. This will make it harder to run away or shorten the gap between you and your opponent. As ranged casters you do not have to get into your enemy's face to attack them. Sion being melee must. "Ignite" is also used to take away that tiny health you were not able to get when the enemy was up close and also aids in defeating champions such as Dr. Mundo or Swain you can heal themselves.