Well, it depends on who you ask.
MG Siegler says that he uses “the 13-inch all the time now as my primary machine. Right combination of size, performance, and yes, retina. “
MG and John Gruber are both of the same thought. I have diametrically opposite views compared to the two of them. I find the 15-inch Macbook Pro with Retina screen is a more complete machine and provides a better bang for the buck. It has superior performance, better visual real estate and has overall a better feel to it.
I tried living with the 13-inch Macbook Pro with Retina for about six weeks. I took it on a trip as well. The weight differential between 13 and 15 inch Macbook Pros (with Retina) isn’t that much (less than a pound.) Compared to the 256 GB 13-inch machine powered by a 2.5 Hz i5 processor, an extra $200 you get a 2.3 GHz i7 processor, more shared L3 cache and a dedicated Nvidia GeForce GT 640M graphics.
The portability isn’t that much an issue, though the only annoying part is that I have to carry a bigger bag to fit the 15-inch machine.
For traveling overseas or when I am out of town in the US for a few days, I prefer something really light as these days, all I use the machine for is blogging and writing longer pieces. I can get by with the Logitech bluetooth keyboard for the iPad, but sometimes it is hard to write for longer periods of time. So, I find it easy to drop the lowest end MacBook Air 11-inch in my bag along with an iPad (or iPad Mini) and still come under the weight of a 13-inch MacBook Pro with Retina. A day trip means I can just leave home with an iPad.
My advice to others has been go with the 15-inch MacBook Pro with Retina.