If aesthetics are important to you, you need an extra port for whatever reason, or you need two really good picture modes (one for day and one for night) then get the GT50. On the ST50, you can only really use Custom and Cinema for Day/Night but Cinema lacks any gamma control and it's a little low (around 2 - 2.1). You generally want your gamma to be between 2.2 - 2.5 depending on room lighting. Typically a lower gamma is better suited for a bright room, but I don't know the brightness capabilities of Cinema mode (I have only calibrated Custom where you can reach upwards of 50fL). I have used Cinema mode for a bit and it looks like it can get pretty bright, so you could use Cinema for day and Custom for night (or vice versa).
As Bill said, the GT50 will slightly outperform the ST50 out-of-thebox, but a calibrated ST50 will outperform an uncalibrated GT50 in THX mode, and both will perform the same after calibration. As for aesthetics, the ST50 is fine in my books.