Delcatty has a wider variety of moves it can learn by TM/HM, but Slaking has better stats, making it more powerful. However, the Truant ability hinders Slaking a lot while Delcatty's Normalize and Cute Charm abilities both have useful effects in battle.
It's really a question of which you'd prefer: a Pokémon with more coverage (Delcatty) or more power (Slaking). Personally, I'd go with Normalize Delcatty and a moveset based on the foes you expect to fight, which is very useful against in-game trainers. If you plan on fighting competitively against other players, I'd recommend Slaking, but there are a lot of Pokemon that work better in competitive battles.