I think most people would prefer the 13" due to form factor, however there is a lot to be said about typing comfort on the 15" (it is far more enjoyable thanks to a rest area for your wrists, while on the 13" it can dig into your wrists). If you are just browsing/watching while it's on your lap, the typing experience may not matter as much, but if you are looking to type a lot on your lap, I think the 15" is a good shout.
I feel the 15" just naturally fits my lap better and the viewing distance while on your lap is best suited for the 15". 13" works better for me on the arm of a couch than on the lap. Maybe the perfect size is 14"? Never tried one