No brainer for me, get the 15" with the 256GB drive. For the same price as the 13" you get a much faster machine on top of having the larger screen which I believe is a definite plus over the 13" (much more than extra portability).
And again how is 2 more cores "much" faster then the 13"? If your getting a base or mid tier 15" all your getting is 2 more cores, a slightly faster iGPU and a bigger screen.