competition between software publishers
...and since the hardware is all but identical, competition isn't coming from Sony+Microsoft, unless you're one of the few people that think gamers will actually buy into Kinect this time round. They'll both get the same games, the PS4 will run them faster or prettier, the XB180 builds will have superfluous Kinect gesture support grafted on (but left unused by players). In a years time they'll have equivalent social network support, and video+media hub functions.
And they'll both have increasingly overpriced software in a world of mobile and resurgent PC gaming, markets that do have actual competition instead of a phony platform war based on theatrics more than actual competition.