Also keep in mind that game performance would not be a good reason to choose a 15.6" 1366x768 display instead of a 15.6" 1920x1080 display. The basis of this misconception is underconsideration of all necessary factors.
For one thing, if your framerates are suffering while running in 1920x1080, you can always step your resolution down to 1366x768 inside the game settings. With a 15.6" 1366x768 display, you cannot make up for the limited screen space that's going to affect everything else you do.
But the resolution itself isn't all: 15.6" 1366x768 displays tend to be low-grade LCD panels with very poor image quality due to low contrast, whereas 15.6" 1920x1080 displays tend to have great contrast and quality. What this means is that despite the blur you get when running games in a non-native resolution, you'll likely find that games look better running in a non-native resolution on a 15.6" 1920x1080 display, than even in native resolution on a 15.6" 1366x768 display. So you really aren't gaining anything by getting a lower-resolution and poorer-quality display to run your games on. And of course, quality differences are there outside of gaming too, so by going with a 15.6" 1366x768 display you're not only killing the usability of your computer in terms of the resolution not letting you fit much onscreen, you're also killing image quality in everything you do.
Game performance with the GTX 660M on 1920x1080 is not terrible anyway; the GTX 660M is more than powerful enough to run many games in more-than-acceptable settings on 1920x1080. But a 15.6" 1366x768 display is a terrible display. You need to be concerned with improving a display that's pretty bad before you start trying to improve upon game performance that's already pretty good.