I think it's funny that people state the 15" isn't as portable as the 13". That's ridiculous. The weight and size differences are minuscule. You can take the 15" everywhere you can take the 13".
The 15" will always be better in every aspect.
13" rMBP: 31.4 x 21.9 = 687.66cm^2
15" rMBP: 35.89 x 24.71 = 889.71cm^2
687.66/889.71 = 0.77
In other words the 13 occupies a quarter less surface area of your workspace. Flip the lid open to use each machine and the 15 is a quarter bigger across two almost perpendicular planes. By most peoples definition of the word, that is not a minuscule difference.
It may not make much difference to the environments you use the machine in, but for some of us that is a big deal.
13" rMBP: 1.57kg
15" rMBP: 2.02kg
1.57/2.02 = 0.78
In other words the 15 is a quarter heavier then the 13. Once again I find the use of the word minuscule inappropriate to describe that disparity.
For me personally the 15 is not better in every aspect, because surprise surprise, some of us don't consider such differences as minuscule

Credit us with having the sense to know what is important to us and what works best for the environments we use our laptops in.
I chose the 13 over the 15 because it was the better package and in particular better in important aspects for my usage. Coming from Windows machines, I'm used to paying more for the smaller/lighter machine, so its a pleasant surprise for once to pay less (not that the price difference was significant in my choice, if the prices were reversed I'd still take the 13 all day long. And have done in the past).