Go with the 15". I was sold on the 13", for "portability" and had convinced myself that I could sacrifice the real estate in the hopes for a more portable unit. Luckily, I read some responses on here pushing for the 15", both for screen size and especially the discrete graphics, and was on the fence (again, I was SOLD on the 13") when I walked into the store. I also was hesitant about the Retina screen, I was thinking an Air would be perfect. Took one look at the 13" Retina side-by-side with the 13" Air, and knew 100% non-Retina was no longer an option.
Had them bust out the 15" Retina and I said "that's the one!"
I spent all last semester (got it January 4) packing this thing around in my bag, and there is absolutely no reason, in my opinion, to give up the screen real estate and the power the 15" has in favor of a minimally more portable 13". Coming from my previous 15" Vaio, this thing is an absolute lightweight. It's faster, runs cooler, is dead silent, more durable (I dropped it about 4 feet when it was 2 days old and works flawless. My Vaio would have been dead), and the screen...well what can you say about the Retina? It's FANTASTIC!!
My vote goes to the 15" Retina. I still have at least 2 years of school left, and plan to carry this thing with me the whole way.