I wouldn't worry about the difference in video memory(1.5gb vs 2gb) but I think the gtx660m is actually better than the gtx670m because its keplar based whereas the gtx670m is fermi based(last years technology)
with that being said if you get the gtx670mx upgrade for extra $75, it should stomp all over the gtx660m.
The size of the screen won't matter because both laptops have the same resolution(1920x1080), the physical size of the screen wont make any difference here.
I'd probably choose the x7 because of its ssd(Makes your computing a lot more fun)
But with a gtx670mx, the x6 would prove to be a more future proof gaming machine.