I'd honestly say that there's really not enough between them to bother picking based on which is better for gaming. Given the huge OCing potential of the 650m, the 660m isn't that far ahead of it so the performance difference you notice will be fairly minor depending on how you've OC'd the GPUs. It's probably better to pick the one which suits your needs outside of gaming since you likely won't notice that great a difference. I'd consider other factors - do you want the Retina display or a matte 17"? Do you want the rMBP's trackpad or the Blade's one? Does upgradability bother you? Do you want an SSD out of the box (iirc the Blade doesn't have one)? And of course, the obvious, do you want a Mac for when you're not gaming? Although if you're after pure gaming machine, it begs the question, why not build a desktop?
I call my windows partition my Xbox partition because it is 100% for gaming. Mac is for everything else. I would never own a restricted windows only pc.