IMHO The Samsung plasma appears as if you're watching on a glossy screen, so it seems to be more bright. On the other hand, the Panasonic is like watching on a matte screen, so it appears to be more dark, thus black is more pronounced. Im sure a lot of people would disagree, but this is just based on my opinion.
Which one is better, it depends on your preference. You have to audition both TV to see what suits your taste, not to mention your budget.