Ok, so I decided that for my latest piano composition, I'd use both the Casio Privia PX-350 and the Nord Stage 2, separately and together throughout the song. The Nord was set on the Fazioli grand piano setting (arguably Nord's best piano sample). The Privia was set on the Grand Piano Concert setting.
The result was quite amazing:
http://youtu.be/HIA4X5KEJ74 One would assume that the Nord would simply sound sooo much better hands down, without even giving it a listen. Well... surprise, the two pianos together - and separately - just sound phenomenal!
The only thing that I wish the Privia could do is give me the same kind of control over reverb that the Nord does. Privia only gives you 4 hard-wired reverb settings, none of which are great. Nord gives you the complete spectrum of control over reverb.
I think this video shows that the Privia can step up to the plate and be used for music composition as well as both studio and live performance, and sound pretty damn convincing!