Recently purchased Sion. Played a few games as AP Sion.

His W is more suited to AP, however his E and R are more suited to a DPS build.

Personally I think his Q is better for a DPS build (stun helping in fights), but there are way more guides for AP Sion than DPS suggesting AP is more popular.

or a mix of the two?

Personally I dont really like AP sion, I prefere to play his as a champion who can take a handful of damage but still dish out alot of DPS. Frozen Mallet is a good item for him I find-alot of HP, some damage and a great slow which helps your team and goes well with your stun.

AP...makes an extreme nice nuke out of his stun and his shield makes it able to farm like crazy which is giving you more life if axe is activated...

well it's up to you how your playing style is...
AP sion is safer than DPS Sion who dies extremly fast...

ad-if your team already have some mages
ap-if your team already have some damage dealers
hybrid-? never tried

Thanks guys i'll take your suggestions into account and I guess i'll just play him depending on the situation.

a melee champion with no way of quickly getting into the fight is usless as AD dps imo so ap much better, yes it makes his ultimate not that useful but with the great ap ratios on stun and shield if you have lich bane and a nice amount of AP you basically have three nukes wich can kill a lot of the squishier champions. I have never tried tank sion but im sure its nice with shield/passive and a nice stun for your team.

ap sion is the incarnation of low-elo

In allmost every case AP is a better choise then AD.
AP sion is allso a really good solo laner and AD is not.

Well to be completly honest, AP Sion is a very good champion and AD Sion is not that good at all.

AP Sion's good early. In team fight, if they target you, your shield is away in seconds, leaving you without the AoE blast.
AD Sion has evil AoE heal. Try AD Sion with Yomuu's Ghostblade.
<------ Killed Tryndamere's with Yomuu's and Ulti many times ^^