Joe,
I just bought a Panasonic TH-50PZ800U after looking closely at the Panasonic and Pioneer units. The Panasonic was a fantastic value and is very well done. I'm extremely happy with it.
If you can afford it, go for 1080p and you won't regret it.
In the Panasonic line, the "800U" model has THX certification, but the step-up 850U model drops this in favor of added equipment that allows you to connect your TV to the internet and download various content directly into your TV. I thought this was stupid as I plan to keep the TV for many years, and chances are, this feature will be obsolete in just a couple of years -- the same way I never recommended buying a TV with built-in VCR!
Anyway I would suggest avoiding LG and go for Samsung, Panasonic or Pioneer if you possibly can.