Well, for one thing, it seems on the Pianoteq, (from that screenshot, at least) that unison detuning is a global parameter, whereas on the V-Piano it is per note (if you want).

This will help the piano sound more 'real', as unison tuning is rarely, even with the best piano technicians, consistent from top to bottom.

I liked Pianoteq2's demos, but overall, I felt that Roland's sounded a bit more like a real piano, better than samples, whereas the Pianoteq didn't QUITE cut it against the best of the sampled sets out there.

PurgatoryCreek.com has examples up of the Pianoteq (no Roland V-Piano, yet...) for you to compare to other sample sets, including GIGA. In fact, you might want to put the MS's GIGA piano up there for comparison...