Well, ranking 50" plasmas is pretty easy, IMO.
The Pioneer Elite Kuro plasmas are at the top, but the PRO-111FD and PRO-101FD are out of your price range and not likely to ever get that low before they are all gone.
The Pioneer KRP-500M monitor (no TV tuner, no stand, no speakers) is lower in price, but still above your price range and disappearing fast.
The Pioneer PDP-5020FD gives up a bit in terms of color accuracy, but is still excellent. Still above your price range though and discontinued, just like all the other Kuro, sadly
Next, the Panasonic V10 series picks up the mantle. A very small step down from the Pioneers in black level performance. The biggest step down is black level retention in lighting (the Panny's wash out a bit and look a bit dull and grey in room lighting) and the Panny screen is more reflective as well.
Don't take those comments out of context though - we are talking about SMALL differences and the V10 Panny plasmas are excellent! The TC-P50V10 is above your price range right now, but it might drop within range by Black Friday, so keep it on your radar!
The G10 series takes a small hit in color accuracy, but it's still the black level champ of the more affordable plasmas. That said, the Samsung plasmas have excellent color accuracy and their black levels are only a very small step down. The biggest issue for the Samsung plasmas, IMO, is their rather reflective screen. Your own reflection is quite visible in normal room lighting if you're looking at a dark scene. Seeing reflections bothers me a lot. This is something you should really see in person. Just turn off the TV at a local store and take a gander at how visible your own reflection is in the screen surface!
So your choice is going to depend upon your room lighting IMO. In a dim room, I would personally probably go for a Samsung over the Panny G10 because I prefer the more accurate colors of the Samsung and the hit to black level performance isn't something you'd notice without a side-by-side comparison.
With room lighting, the Panny does wash out a little, but it's screen is less reflective and that's a bigger deal to me.
If the V10 is within reach though, that's the one to go for, regardless.
Good luck!