I'm going to throw the cat in among the pigeons here.
This is very much like the hi-fi dilemma. Mucho spend for minimal difference, once you get above a certain level.
Our main set is a Samsung LED. I agree that the Panasonic plasma provides (slightly) better image quality, but no way is it 'night and day'. I spent a lot of time setting up the big Sammy properly, and the difference that made, IMHO, is greater than the difference between it and another screen.
Not to mention that the broadcast medium is pretty important. Years (many) after its introduction, you might want to take a look at exactly how many true HD channels are broadcasting. Equally importantly, how many are broadcasting stuff you'd actually want to watch.
The vast majority of stuff I want to watch is Blu-ray or DVD music concerts or films. About zero of them are recorded in high def. A big screen brings more involvement, but not better quality per se.
Ours is a 42". If I ever change it, I will go for a 55 or 60" LED. Cheaper (by far) to run than a plasma, and bearing in mind the media, so close to equal as to not matter.
Spend the difference on the audio side.
It's what I did. Concentrating on the fact that it's mainly doing 'in your face' stuff, I bought a sensibly big pair of B&W floor standers for the front, a pair of B&W 601s for the rears and a really decent centre. The sub is £100's worth, because mostly it just does explosions and helicopters and the like.
Priorities...