I just upgraded from a 13" MacBook Pro w/ Retina to the 15" version. Key Reasons:

- Display Space: Even when set to the highest possible virtual resolution, there is still noticeable more space on the 15" version. It's still way worse than a 27" display, but the difference makes me more productive.

- RAM: Retina eats RAM. I could run several browsers, VMs, editors etc. just fine on a 4GB MacBook Air. When I upgraded to a 13" MBPr, I was constantly running out of RAM, the machine began swapping and became barely usable. With the 16GB 15" MBPr, it's a very noticeable difference. I easily run into >8GB RAM usage. In addition, when there's available physical RAM, OS X caches some stuff from the SSD which speeds it up further.