The LG should be capable of a brighter image and possibly better color accuracy if properly calibrated, but the contrast ratio will be much worse. LGs also have bad color banding, and I'm not sure about the newest models, but they've had bad dithering in the past. Pick your poison. I personally would rather spend extra for the ST50, but choosing between the two, I would pick the UT50. I own an older LG, so I already know how much some the LG issues bother me.
As chunon said, if you are interested in the UT50 and don't care about 3D, then it would make more sense to buy the 60" U50/U54. It's very inexpensive -- probably the same or less than the 55" UT50.