You'll get better answers from others as I'm sure they've done a more thorough research that I did but FWIW here's my opinion.
Budget wasn't a concern this time around for me, I was contemplating picking up the Sony 65HX950 (in Canada, it's $6K) which is 50% more than what I can get the 65VT50 for. I compared all three plasmas and for my own use and preferences, I ended up with the 65GT50.
The ST50 wasn't an option for me as I wanted THX mode and in hindsight I don't regret it. THX Day and Night modes on the GT are awesome, that's all I use now even for gaming. The 4 HDMI ports was also a must, I'm already using three and will be getting an AppleTV soon to occupy the forth port.
The GT's dual-core processor also makes Viera Cast/Internet Apps really smooth, on the ST I found them to be a little sluggish. Also, the aesthetics of the GT's bezel won over the wife factor. The ST looks like cheap plastic sitting next to it.
Having said all that, if none of those are important to you, then the ST is a better choice value-wise. They have the same panel so the ST can produce as good an image as the GT can (with calibration).
At one point I did compare the VT and the GT but I wasn't planning on getting a pro-calibration done so the added pro settings/tools on the VT were meaningless to me. Also the VT has an overall darker image (for slightly better blacks) so I thought in the long run if I ever place the TV in a bright room the GT might perform better. For the price difference I just didn't see any added benefit to the VT (I was able to get the 65GT50 for 2/3 the price of the 65VT50).
I think any of the three is a great set, it just depends on what features/benefits are important to you. If you get the chance to look at them side by side, do it.