Had both the 42" UT30 (2 sets) and now a 46" ST30 and can advise the following differences:
ST30 has a good A/R filter, very good at rejecting light in the room (this means blacks and contrast are maintained with lights on) the UT30 does not
ST30 has a dark grey bezel & stand whereas the UT30 is all black
Contrast and brightness of the ST30 is better, it has the latest neo plasma panel the picture is slightly cleaner as well, the UT30 seems to use last years panel
Black levels are similar in a pitch black room although I find that the filter on the ST30 gives the blacks a slight blue tint so they appear slightly lighter than the UT30
ST30 has built in fans the UT30 doesn't
If these are similar prices then the ST30 would win out for most people because when you view in the daytime the picture is better and contrast is held even with lights on (have a look at the ST30 in a showroom for how well it holds its black level)
For me however the filter is irrelevant as I only view in my dark batcave
The ST30 IMO doesn't look as good as the all black UT30 in my room but it does seem classy and when off the screen itself is black and contrasts well with the dark grey
Both my UT30's had faults (vertical lines and 3D bug) btw and the ST30 has green splodges and fan noise (& 3D bug) and I'm currently having it replaced by JL
For overall picture quality I would say the ST30 wins though
Apologies for the long post but the UT30/ST30 differences Q's keep coming up