Wow, they look pretty impressive-- "infinite" polyphony, T2L technology, and a combination of sampling and modeling! Either model should certainly work fine as a MIDI controller, in addition to being a digital piano.
My only thoughts about using it as a MIDI controller is that it doesn't have many controls the way that most dedicated controllers do-- but you should be able to buy a separate mixer-style controller to use with it if you need more buttons and dials and faders to fiddle with.