no idea about the 720, but ps4 is set up really well. they have a chip design that is easy (relatively) to upgrade. they can just add faster spe's and more of them. they add a well designed waggle mote. boom you have a ps4 that is out early, cheap, and runs like old hardware but is still vastly improved.
on the other hand the wii 2 wont upgrade well (maybe). they will have to improve the system by so much that the hardware would change a lot, taking out a lot of the cost effectiveness.