Hi there.
I will make it really simple for you (big starcraft 2 fan, I play it regularly on many apple machines, from imac mid 2006, to various macbookpros and iMacs i7).
It breaks down to this:
If you are going to play on the laptops screen you can go for the 13 inch, no problem. You will be able to play it at the native resolution of the 13inch screen with relatively high details.
Instead, if you want to play it on an external monitor at 1900x1080 then you must go for the 15 inch.
Alternatively, you can play it on the 13inch on an external screen but on a relatively low resolution (no more than 1400x900) with mid-low details or the framerate will drop dramatically.