I stumbled across this old, rather interesting thread and decided to add my two cents even if time has passed.
I just donated my Yamaha CLP115 (2003) to a music school, and have played with PTQ 8 demo quite a bit. As much as I cherished my Clavinova, I think there's no comparison whatsoever to PTQ when it comes to sound quality, realism, depth, etc. The Clavinova's timbre was very nice, but the difference is still huge.
Even though--after much tweaking and tinkering (and possibly getting accustomed to it)--I start to like my Roland F107's partially-modeled Supernatural sound better, I still think PTQ has an edge in terms of roundness and presence. PTQ basses are pleasanter and the trebbles are have more spark. I ignore the exact specs of most digital pianos sound engines, but my impression is that most VSTs sound better.
This is both baffling, since the main feature of DPs should be a great sound, but also makes sense, since if they weren't better, VSTs wouldn't have any reason to exist.
As for sampled libraries size vs modeled software, my take is this. While libraries might be comparatively big (say 5/6GB), SFZ players are rather economical. I made lots of testing, and PTQ consumption of both CPU juice and RAM is always higher than even the biggest SFZ libraries I play.
Just an aside.
As anyone new to pianos, I'm a sucker for the real thing. The other night, out of curiosity, I wen't to try a 1926 very well kept Cable baby grand for sale. I was quite excited, never played an acoustic before. Of course, it was quite a shock. Even compared to the Roland, it sounded disappointing. But I'll keep trying to play other acoustics to get a better perspective. For now, digitals are still the way to go for me, but some day, who knows?