In the end it comes down to personal preference... the 15" is a very stout machine, and the dGPU makes it somewhat decent for gaming.
The 15" is very portable, but the 13" is even notably more so. It's light enough to pick up with 3 fingers.
My main machine is a quad-core Mini, but there's no difference in day to day use, and my dual core i5 runs Logic and Aperture just as well as my quad-core i7.
The only two limitations of the 13" are GPU (integrated only, but it makes a difference in fewer apps than you might think), and the RAM which maxes out at 8GB. If you plan on running some VM's then you'll want the quad-core 15" with 16GB ram.
If you're just running apps, then 8GB and dual core will be fine for a few years.
Screen real estate is a non-issue, because the 13" can run in the same scaled resolutions that the 15" can, if you use a 3rd party app like QuickRes. I normally leave in "best for retina" setting (1280x800) for email and web, and toggle up to a higher resolution for Logic or really large spreadsheets.
I have a base model 13" rMBP now but will likely trade up to a Haswell model with a larger SSD when they're released. I thought the 128GB SSD would be fine but then Logic X came out with its 30 gigs of downloadable content.
