Ahoy Danest!
I was in the exact same spot, regarding what Mac to get for programming and mobility,
After much consideration (and testing on both the 15" and 13"), i must conclude that the diffrence is really not that big to be honest. I have been using a 13" for the last years, and i didn't find any need to upgrade the screensize.
If you have a 13" allready, you will find no problem using it for programning.
Besides - the computer can be hooked up to huge screens for, when you are home or just have the posibility. 🙂