The (relatively) new full hi-def Panasonic 65" plasma is a real stunner. I have a Pioneer 50" plasma (Japanese model 505HDL). I haven't seen the 60" Pioneer (that Velo62 refers to) even though I am through that kind of store quite often for other reasons. I'll have to check the next time I'm in one - probably late today :^).
Now that both Pioneer and Panasonic are full hi-def at the top end models, there is really not much difference. The Pioneer has a slightly better front glass I find, the Panasonic has a bit better contrast ratio, and the color fidelity choice is really one of personal opinion. The Pioneer reportedly puts out slightly less heat, and weighs a bit less (partly because of front glass structure). Speaker quality on both kinda stinks. I happen to like the Pioneer colors better but the real difference in my choice was that I got a better deal on the Pioneer than was possible on the Panasonic (temporary inventory issues at the particular branch of the chain where I bought).