Which one of these is better for developing pecs, wide or narrow dips?
You'll need to activate your core. When you're in dip position (body totally upright) lean your torso slightly forward, and pull your legs slightly forward. You're body should look like a < (only not quite as pronounced).
It's challenging, and difficult to express without demonstration, I'm sure someone at your gym will know exactly how to demonstrate for you. Good luck!