I've also been trying to decide between the two. There is also one thing you should know to consider that many don't mention. Here's the a quick summary:
1 - Viewing the ST50 at eye level in a bright room, its clearly a better picture than the UT50. Its louvre filter is very effective.
2- Viewing the ST50 off-axis ABOVE or BELOW eye level, its picture begins to dim and gets much worse at a severe angle. This is a by-product of the louvre filter. The picture is fine at all horizontal viewing angles. The UT50's picture is consistent at ALL angles.
3 - So - if you will have the TV mounted high, or you casually lay on the floor and watch TV, you may be troubled by a darkening of the ST50's screen. If you can't control light well, the UT50's excellent picture is negated somewhat by screen washout.
In my case, the TV's for the basement and movie watching. I can control light well and tend to lay on the floor, so I'm leaning toward the the UT50. In controlled lighting situations, its as good as the ST50 to most eyes. I would lean toward the ST50 if its screen didn't dim off-angle vertically, but unfortunately that's not reality. Maybe they'll tweak the louvre filter for 2013!