Portability for me cause of work
however there is no reason you can't have both.
Nintendos supplementary unit patient is a hint of what may come next gen.
Just need the portable console to be say PS4 level and the dock to be able to add say another 4-6tflops to the system. For that they need a fast port that can handle offloading the tasks to an external gpu or perhaps two portable gpus in the dock.