13" or 15" Retina MB Pro

I'm in the market for a new laptop, and I can't decide what to get. I'm torn between the 15" Retina MB Pro and the 13" Retina MB Pro.

My current laptop is a 2010 13" MB Air, and it has reached the end of its useful life. While it can handle basic tasks, I need something more powerful. Most of my day revolves around Sparrow, Safari, and MS Office. On the weekends I like to mess around in Lightroom, but I'm pretty much a photography noob. I should note that I have to carry it everyday in my backpack to and from work for 20 min, so weight is a consideration.

I like the 13" because it is cheaper, lighter, and smaller. I like the 15" because it is more powerful (CPU & GPU) and has a bigger screen.

Which one should I choose and why?