While the dimensions and weight are facts, the "feeling" of portability is just that. If you have a tiny briefcase or backpack or cargo pants pocket that ONLY the 11" will fit, well there you have it. But at just over 9 ounces heavier and one inch wider/longer, the 13" fits is just about any "normal" briefcase or backpack. And being just as thin as the 11, you can slip it in there just as easily.
So in terms of portability, I see them very, very close together. A lot closer than some posts here make it seem.
And my advice to people on the fence is that you will be USING it a lot more than you will be CARRYING it....

Choose accordingly.