Use the glaive for faster channeling, healing yourself in pk, getting attacks out fast, etc.
Use the attack weapon when you are able to do combos or have the upper hand in pk.
Some wizards only use the attack weapon while others use both or more. But the reasoning why is what I typed above.
You'll get the hang of it after a little bit, it's nothing too much. Having a defense weapon is good in pk too as well.