Here's my decision from active research:
When talking about top-rated Plasma TV's, seeing them side-by-side in a showroom isn't going to tell you much, so I was reduced to specs, reviews, and physical appearance.
I bought a P50S60 thinking that I don't care about 3D and I have room with moderate lighting. The P50S60 does have a great picture even if you do nothing more than put it on Cinema and then increase the contrast. (I prefer to get used to a TV for a week or two before doing a lot of calibration) - Especially compared to LED/LCD TV's.
Unfortunately, the S60 is a mirror and you must control the lighting very carefully. The picture is bright, and my room is not, so that's not the problem. The problem is that whatever lighting you have must be indirect and carefully controlled and then I still see reflections of the room on the screen during dark scenes - which sort of kills the point of having the great contrast and "inky" blacks.
The Samsungs in my price range are even more mirror-like!
If I had the luxury of a home theatre room where the environment could be sufficiently controlled, I'm sure I would've been happy with the S60, but I don't.
So, I sent the S60 back.
I looked at the VT50, GT50, S64, and the ST60 and decided on the ST60 - which will be delivered in a few days. Here's why:
Year over year, Panasonic has made improvements in picture quality and signal processing.
The S64 is the same panel as the S60, with a louvre filter - which is a low end filter.
The VT50 is simply too expensive - especially in a time period when significant advances in Plasma technology seems to be so rapid. I didn't even consider the VT60.
Initially, I thought that surely, the heavily discounted upscale GT50 would be a better TV than the ST60.
The P50GT50 and the P50ST60 are indeed similarly priced. The P50GT50 is a beautiful TV to behold, but that's not important to me.
Everyone seems to agree that overall the ST60 has a marginally better picture than the GT50, but that the GT50 has some subtle advantages and additional features.
So, it came down to this: Given the year over year improvements in Panasonic Plasma's, I decided that going with the newer model and slightly newer technology of the ST60 outweighed any possible advantages of the upscale GT50.
However, I'm sure that the GT50 is a great TV at a great price if its feature set is preferred or if the physical appearance of the GT50 sets your heart racing. If I had found the GT50 for a lower price than the ST60, I probably would've bought it, but I didn't.
So, it's the P50ST60, but I'm sure that the P50GT50 wouldn't be a mistake for anyone who values the physical appearance or additional features of the GT50 over the ST60. (Even though I said the physical appearance isn't important to me, the GT50 is a beautiful TV to behold!)
Just remember, these are all great TV's and you have to live with them a while to really tell which you prefer and in any one category, they are so similar that the differences mostly boil down to personal taste and budget.