Option 2 if PQ is your number one concern. A calibrated ST50 will look better than a GT50 in THX mode. THX mode is good for a preset but it's not great compared to a calibration.
Option 3 is good if you have a calibration disc AND a meter. A calibration disc by itself will only get you so far. To do a full and proper calibration, you need a meter.
I think the only time I would recommend a GT50 is, you prefer the styling, the extra features appeal to you and/or you're not getting it calibrated.
THX Cinema and THX Bright room are both more accurate out of the box compared to Cinema and Custom, but both Cinema and Custom mode can be calibrated to look better than the THX modes.