Yorick Ap or Ad S5 ?

AD.

AD Tank pretty much. You only have one AP ratio, your ghouls' damage is based on your AD, and your sustain is as well.

If it helps, I typically build something like the following when playing Yorick:

Alternatively, you could take a more offensive mid-game build like the following:

Skill Order: EQEWER, R>E>Q>W Starting Items: Crystalline Flask, 1 or 2 Health Potions, 2 Mana Potions

Yorick's E and Q scale with AD and his ghoul's do physical damage. However, his E does magic damage when it's cast and so does his W which happens to scale with AP.

The only viable way to really tap into his magic damage is to build if you don't need other boots (you can also take hybrid penetration runes). Especially since in lane you're spamming those 2 abilities to harass.

yorick falls off soooo terribly late game RITO PLZ

AD and fairly tanky. Dunno why people hate on Yorick, his teamfight is amazing and his lane phase is probably the best in the game.