This is an easy decision. The Panasonic GT50 is going to blow the LG out of the water for picture quality. It will have better screen uniformity, better blacks, better color accuracy, no motion blur, no soap opera effect (unless you want it). Sharp is the only game in town for 70" TVs. There are some tradeoffs to the increase in size. The picture is good, but not great, and Sharp's degrade quickly as viewing angle increases. I haven't confirmed this for all their models, but it appears that the de-judder system in 2012 models cannot be completely turned off, so if you don't like the Soap Opera Effect, I think Sharp would be out.
Most Panasonic plasmas do not have buzzing you can hear beyond a foot, especially the 2012's. Get the TV at a place with a 30 exchange policy, and exchange it if it does.
I would only consider the LG or Sharp if your room is incredibly bright (which isn't very comfortable viewing). The 2012 Panasonic plasmas are brighter, but still not quite as bright as many LCD TVs. There is a daytime THX mode now, which is bright enough for most situations, and you can make plenty of adjustments on the GT50 to get it the way you want.