Getting a Mac for Gaming

Currently running around with the macbook air on my back, but I've been counting the days down.Since making it my primary computing house, it was inevitable gaming was to be tested. Once I tried civilisation 5 and Call of Duty, it was clear to see my hopes were mis-placed.

Looking at the specifications I've considered two viable options, base 27-inch iMac or otherwise replace my macbook air, the lighter-than-a-book machine I've called home for two years, with a base-model 15-inch macbook pro retina?

Verge-ians I need your advice! Gracias :)