Which new 13" rMBP?

I have a 2011 27" iMac Core i7. I presently edit PremierePro CS5, but want to switch to FCP X.

I need a new laptop. My old one is a 2008 15" MPR, that I used to edit before getting the iMac. I decided on a 2015 13" rMBP.

Although I plan to do the heavy editing on the iMac (I am an advanced amature and do this for fun) I want the new rMBP for portability and if I feel like it to edit Lightroom, PhotoShop and FCP.

I understand that the best result is a 15" MBP with discrete graphics, but this will NOT be my primary machine, abut a might as well possible editing machine.

So I am thinking at most of a 13" 2.9 with 16 gb ram, although I aam open to the 2.7, but not the 3.1.

Appreciate your input,

Harvey