The GT50 has more inputs, better styling, dual-core processor for apps and THX Cinema and Brightroom (and THX 3D). It's pretty much unanimous between the pros that have seen both, that the extra shades of gradations makes little to no difference. The main benefit of the GT50 is the THX modes... they offer decent out of the box performance (slightly better than the ST50), as well as two good modes that can be used for darker rooms and brighter rooms. The ST50 is limited to Custom mode and Cinema mode if you need two modes for day/night (or dark/bright). Custom mode can be calibrated to perform just as good as the GT50 (even a calibrated GT50) but Cinema mode is a bit more limited in its picture controls, specifically there's no gamma control and the highest it can achieve is ~2.11 (which is a bit on the low side). So basically, both sets at their peak performance are more or less the same, it's all down to whether the features and additional modes are worth it to you.
For me, the ST50 was a no-brainer. I'm a DIY calibrator and I only need 1 picture mode. Given your options, the ST50 package is a much better value and would be my choice hands down. But then again, your needs might be different.