For photo editing, I would go for as much resolution as possible. I use the 15" Macbook Pro with Retina, with 2880x1800 and that makes it usable. I would prefer a 17" but that is not available. My wife has the 13" Retina version, but I find that too small.
The higher resolution screen gives 4,7 mega pixels (full screen) resolution, which is more than 2 times the other. in my view that is worth to paying a little extra for.