I do not game so I don't know how demanding these games are. But I think the 13" would be best because it doesn't have a discrete GPU. That hurts you for gaming, but if it is casual that might not be too much of an issue. When you boot into Windows, it is coded on Macs to fire up the discrete graphics,resulting in lower battery life and more heat. Since the 13" doesn't have one, it will run on the integrated chip-- making the computer cooler with better battery life, ideal if you spend a lot of time in Windows.