With the retina display, that gives you a good compromise. The higher resolutions give you greater desktop space.
I found the 15" a bit too heavy, and not particularly portable. I will be able to get a better idea when I have had a little time to play with it.
Out for delivery today!
13" should be fine for out and about usage and photo editing.
It's essentially what I'm going to be using it for. The odd bit of video editing from GoPro footage, but nothing in-depth. If I were really struggling, I would connect it to an external screen.
Chances are though, if I were doing that, I would be at home, and have access to my PC. So that becomes a non-issue for me.
The 13" to me made more sense, as it's a nice compromise, plenty of power for most tasks, and more portable than the 15".
I'm not going to be doing any gaming on it, so don't need the dedicated graphics power of the 15" beast.