Best level 2 spell for a Gish warlock

Pact of the blade devil dwarven warlock, fighter 1/warlock x. Got Hex and Armor of Ag, with charm person for non combat. Mask of many faces as the invocation. I am the tank. I was debating between misty step or mirror image, but then I have 3 combat spells and only 2 slots. So the questions are :

1. do I swap out a spell?
2. should I get misty step, mirror image or invisibility?