Slaking-Even though the ability is garbage being truant. Has a decent speed, a monsterous attack, and decent defense, plus I think you can get it earlier. Slaking has amazing health. It can cover it's weakness it can learn Aerial ace. Slaking has amazing coverage.
Delacatty-It's only decent stat is speed, It's got slightly better type coverage. None of the type coverage is great on it. It has all of it's stats lower than Slaking, Delcatty is just awful.
It is clear Slaking is better.
It's speical defense is it's lowest stat
a ton damage will be delt
more raw power
coverage for fighting
An ok physichal attack
a strong physichal move
It is super clear that you should pick Slaking