50VT20 brightness is a bit low especially in 3d. But should be sufficient.
VT20 has floating blacks.
3D is not as good as 2011 models (Or so they claim).
On the positive note :
VT20 has ISF Day and Night mode (Picture control options that you cannot even begin to imagine, It has more parameters to tweak than a mid range SLR in Manual mode
).
THX mode.
A slight edge on PQ.
Flagship Plasma of 2010.
ST30 has limited video controls, but as I have said earlier, things look damn good out of box settings (And confirmed by many calibrators worldwide).
Some of the reviews said that "True Cinema" Mode is very close to THX level settings found on Panny that feature THX.
Variables to consider :
VT20 has Rising blacks, which means black levels will rise as panel ages. However, the rise is not dramatic, and it still will have good Black levels.
2011 Panels have slightly higher black levels than 2010 models. But they claim the black levels will be stable. But since xx30 series are new, no one can confirm this yet. (Overall outlook is positive though).
Overall, you cannot be wrong if you chose either. But VT20 has been discontinued. But Panny will still offer 3 year warranty. I do not know what they will do, if they have replace this panel due to some fault. Having said that you will Love both these gems. It all boils down to features you want.
ST30 -> More Features, Better 3D (?), Slightly higher black levels, Stable Blacks(?), Fluctuating Brightness (Subtle, can be fixed), Limited video controls
VT20 -> Deeper Blacks, inferior 3d (?), Lower Black Levels, Rising Blacks (Though not as bad 2009 models, subtle), Floating Blacks (As far as I know, no fix). Plenty of video controls.
I hope, this helps you weigh your options
one other point, while i went to place the order the salesman was very keen to sell 50VT20 to me, he had a fresh stock and it came with 2 3D glasses and it was THX certified. i guess he offered me 1,06,000. Now since the revised offer for 50ST30 is 1,03,000 which would be a better deal?.
i guess there were few bugs reported in VT20 which was corrected in ST30....