If you're already set on a Retina MBP your only remaining choices are amount of RAM and storage. Get the greatest amount of each, because neither one can be changed after you buy it.
You have a choice of either a 2.4 or 2.7 GHz CPU. It is unlikely you would perceive a difference between the two unless you were to compare them side by side.
Bear in mind there is no optical drive in the Retina MBP. If you need one for your work, you can purchase an external SuperDrive for $79.