Renekton deals more damage and does very well with someone who can combo his moves at the right time. He has no mana, but you have to be up in someone's face to be able to do anything.
Yorick has much more range and can create mayhem on a wide scale during a team fight. He is also less susceptable to stuns than Renekton. Yorick DOES have mana problems if you don't have any mass-mana item.
I have more fun with Renekton, but Yorick is much harder to counter.
Renekton is a gamble; hit or miss. Feed or carry. If you're in-between, you play a lot like Garen: MAYBE worth focusing, but best to ignore until the clean up.
Yorick is much safer. He's hard to carry with, but it's also much harder for him to lose his lane than Renekton.