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, Radius 90HD; Martinlogan Dynamo 700, Marantz SR5006, PS3, Oppo BDP-103D
2ch Sony KDL-32W650A, Sony BDP-S1000ES, Marantz PM8004, JVC T-X3 tuner, Monitor Audio Silver RX1, REL T3, Apple TV, Peachtree Audio DAC•iT, Sennheiser HD598
lr Panasonic 50" ST60, Sony BDP-S5100, Apple TV
pc Energy CB-5, Audioengine D1, FiiO A1