It seems to me that you are leaning towards a plasma and I think you would be making the correct choice. I have no idea how much these new LCD based 4k screens cost, but the inherent drawbacks of the technology (LCD) combined with the fact that it will be a few years still before this emerging technology (4k resolution, HDMI 2.0, 60fps video, etc) has much content to watch on it.
You say that you saw a difference with upscaled 1080p video but how close were you? In a showroom setting standing close up to the screen you would certainly see a difference. But sitting 7 to 10 feet away at home would you really appreciate the difference, especially just with upscaled blu rays? When people judge picture quality, the order of importance regarding its various aspects usually goes like this: 1. Black level / Contrast ratio 2. Color Accuracy and ability to calibrate to industry standards (grayscale and gamma tracking, etc) 3. Resolution
Resolution is usually a less important factor in our perception of image quality. At 50 - 65 inches, the other advantages of plasma would definitely outweigh the lower resolution in my opinion. If you put the Sony 4k LCD in a dark room against a VT or ZT60 or a KRP-500m like the one I own, I can assure you you would not feel so favorable towards the Sony.
So my vote is for a plasma. If choosing between the VT60, ZT60 or F8500 it depends on whether or not you would need the additional brightness offered by the Samsung. If you don't need a very bright picture then the improved black level and overall superior image of the Panasonics would be better. As to whether the ZT is worth the premium over the VT, that is for you to judge. I think you would be happy with either.
Another option I should always mention is that I keep hearing about people coming across nearly new Pioneer Kuros for very low prices. There is a risk, but if you find a mint 600m for example it would be better (if only slightly) than all of the other displays.
If you don't want that kind of risk and want to buy a new tv, then if you are budget conscious and want the best value, get the VT60. If you are an enthusiast and would appreciate the small refinements and advantages, go for the ZT60.