The GT50 has a wider array of picture adjustments/settings and a different design; aside from those two things and THX, the panel is the same and picture quality is nearly identical. If you don't like to tweak with settings and don't plan to have your TV calibrated, the THX on the GT is very nice to have...but I wouldn't base my entire purchase on it. I'd take the GT anyway based solely on the fact that I'm not too keen on the ST's design, but if you like the frame, but that's purely subjective.
The smarter money would be spent on the ST50, and spend the savings on other toys.
ht Panasonic 60" ZT60, Monitor Audio: Silver RX6, RX Centre, Silver RX1; Martinlogan Dynamo 700, Marantz SR5006, PS3, Oppo BDP-103D
2ch Sony KDL-32W650A, Sonus faber Toy Monitor, REL T3, Marantz PM8004, Sony BDP-S1000ES, JVC T-X3 tuner, Apple TV, Peachtree Audio DAC•iT, Sennheiser HD598
pc Monitor Audio Radius 90HD, Audioengine D1, FiiO A1