From what I understand the 4K JU6500 and JU6700 are only 60Hz refresh rate which may interfere with watching high speed programs such as sports and also not properly displaying movies. I like your choice of the JU7500 but I've also heard that the Sony's do a better jog of upconverting the image to 4K. This years Samsung's will also accept a proper 4K feed from your computer while the Sony's won't, so if you plan on connecting your computer to your TV, Samsung is a good choice.
I've got a last years Samsung and the smart features are pretty sweet.
You can check out this site for good owners feedback. I don't know enough about the Panasonic LCDs to have an opinion but I like my plasma a lot.
http://www.avsforum.com/forum/166-lc...anel-displays/