Slaking is just a lazy, chunkily designed dick of a Pokemon. That ability Traunt isn't there because the guy is slow or something. It has 100 base speed! No, the reason why Slaking only acts every other turn is because it doesn't WANT to act every turn. And it looks like someone molded a bunch of clay into roughly a humanoid shape and called it a Pokemon.
Slowking, on the other hand, is a stoic badass that takes its time not because it's a dick, but because it's in a state of zen where it can block out the evils of the world, and strike back with avengence.
It also has Regenerator which is the bitchin'-est ability ever.
Slowking all the way.