Going from the ST50 into the GT50 you gain the THX certification which is nice, and in turn the THX mode which is also nice especially if you don't plan on calibrating. And you're supposed to be getting more shades of gradation, IE better black levels. And you get Bluetooth built into the GT which is nice should you ever use the web browser because you can use a Bluetooth keyboard.
Going from the GT50 into the VT50 you're getting again more shades of gradation for better black levels and you're getting the best of the best, cherry picked panels. The VT50's touted by most who've reviewed it or tested it to be the best TV of the year and just that much closer to the prestigious Kuro's . And the VT comes with an auto calibration feature which is a full 10 point. That to me alone is an awesome feature if you don't wanna fork out the $ for a professional calibration or if you're not certified and equipped to DIY
All of that being said, they're all great TV's, the ST50 included, and I would say for some people, you're not going to see a huge leap forward in terms of performance. Just a matter of pickin your poison and what makes sense to you. You'll be happy with all of them. They're spectacular TV's
For what it's worth a friend of mine has the GT50, which has been fully calibrated, and it's a face melter. The thing is an absolute thing of beauty.