Slaking is better, period
Slaking has the highest attack stat in the game, higher than all of the Weather Trio and ontop of that its move-pool is wide and having great special attack makes it being able to use that to its advantage. Its ability Traunt does make it a bit lacking but trust me you can get around that and you will have one of the best if not the best psychical sweeper in the game
@ Silk Scarf
This move set will be your key to sweeping through a majority of the E4 especially Febes entire team of Ghost Types.
Delcatty sucks, period
Horrible attacking stats, mediocre bulk the only actual good stat it has is speed which is only what it has going for it, you could argue that it has a wide move pool but so does Slaking and its stats aren't terrible unlike Delcattys and so do other Normal types such as Exploud, saying that it has access to some good status moves does not justify its horrible stats.
Although if you want to use it here's the moveset...
Silk Scarf @
Hopefully, this helps :D