I would say it's down to your preference... I dont think there's any performance advantage except for the fact it can squeeze more framerate due to lower resolution.

14" is smaller, easier to carry around... screen is not full HD so you will not be able to play games on 1080p though I dont think you really need it on 14" screen... in result, better framerate...

15.6" is bigger, might be a little heavier to carry around but still not that heavy that you can't move around. It has extra num pad on the keyboard, 1080p screen though I find that 1080p vs 720p isn't much of a difference on the screen (i mean pixelated stuff due to stretching)... could be just my perspective... but I always game on 720p resolution for the better details quality without sacrificing much of framerate.

so really, it is actually down to your preference... what kind of usage you are looking at... you like big screen? go for 15.6"... need to be mobile lot more often? 14" is the way to go...