The 13" rMBP has a usable real estate of 1280x800, which is just marginally better than the 1376x768 on low end PC laptops. However, if you want to increase the screen real-estate, you can drop out of "retina" mode and into a scaled 1440x900 or higher effective resolution - and going over 1440x900 on a 13" screen is, IMO, too small.
At that point, you may as well as buy a Macbook Air though. They have the 1440x900 resolution natively, which means less stress on the GPU which means less UI lag.
The 15" rMBP has the same issues, although it has a useful resolution of 1440x900 out of the box, so you can pump that sucker up to an effective resolution of 1920x1200.
Personally, I would go with a macbook air over a 13" macbook pro, as the MBA is going to be better in every aspect except raw power and 'retina-ness'. On the other hand, if you want the larger screen or want more oomph to your computer, go with the 15".
While the OP has made his decision I thought I'd clarify some points, particularly as I see such comments posted often and I think they are slightly misleading.
The MBA's display has a native resolution of 1440 x 900, rMBP 13's display has a native resolution of 2560 x 1600. Setting the rMBP 13 to 1440 x 900 will mean the machine will render to 2880 x 1800 and then downscale to native (2560 x 1600). As such the two images will NOT be the same, the UI will be scaled to the same proportion on both images giving the same "real estate", but the rMBP has the potential to show considerably more detail.
Any textures, images, etc and even text (while not "retina") will be able to take advantage of the extra pixels, presenting more detail to the viewer on the rMBP 13. To illustrate this consider the following example:
1) You have both machines set to 1440 x 900
2) You have the right half of the screen displaying a 1200 x 1600 image
On the MBA that image will be downscaled to 720 x 900 (pixels on half the screen) throwing away two thirds of the detail in the image (648,000/1,920,000). On the rMBP13 set to 1440 x 900 you will see the full image (as half the screen has enough pixels to display the whole image).
I don't see any significant UI Lag to justify that trade off. But even if I did, I'd still take the rMBP display every time. Any Lag will be due to SW issues in any case, as the Iris is more then upto the task for anything but gaming (for which either MBA or MBP13 are a compromised choice), therefore as "retina" displays catch on we should see improvements in this area.
Its undeniable the MBA has its benefits (battery life, weight) but it is incorrect to assume the retina screen running at 1440x900 is just the same as the MBA screen therefore rendering it a mute point in the comparison. The retina screen is definitely a major plus for the MBP no matter which resolution you run it at. And given the display is the primary feedback interface between you and the device I think thats a very important consideration.