If you search around the forums a bit, you'll see that the differences between the ST50, GT50, and VT50 are very small, especially around the PQ side of things. The GT50 and VT50 have the THX settings which are actually very good and pretty comparable to most calibrations, but if you plan to calibrate the TV yourself or have someone else do it, then they are useless to you. The VT50 has the Infinite Black Ultra filter which is supposed to give deeper blacks although I didn't see much of a difference in the store between the GT50 and VT50. Besides that, there is not much else different between the TVs.
Many people have said that the ST50 is one of the best TVs you can buy for the price right now, so you may want to look into that. I ended up buying the 55" GT50 as I was unable to compare the GT50 to the ST50 in the same room at the store and didn't want to risk anything.
So with that being said, I'd say go for the 60" GT50 if you are stuck between those two only.
Denon X4000 | Paradigm Studio 60s | Paradigm Studio CC-590 | Dual SVS PB-2000s | Sony VPL-HW40es | 110" Seymour Ambient Visionnaire 1.3 | Intel NUC i5 HTPC