MBP w/RD 13" or 15"

As a developer, which one would you consider?

What are the advantages/disadvantages of owning either?

I personally like bigger screens. I've always had 17" laptops. But lately I've grown tired of lugging around megatons of weight. But, at the same time 13" seems too small. What are the other factors you'd consider before buying either one of them.

p.s. I'm also an Apple computer virgin. This is going to be my first.