Which plasma TV is better for HD videos - Panasonic Viera TH-P42X30D or Samsung 43 D450?

Panasonic and Samsung are both highly regarded manufacturers of television equipment. Both consistently produce good quality displays and there is little to choose between them.
The models mentioned both have a 768 line display. Although they will both handle HD signals of up to 1080 lines, it is important to note that the final display will use only 768 lines. The incoming image is processed to convert the full 1080 lines to 768 so it will properly fit the screen. The quality will be good, but it won't be at the full resolution of the incoming signal.

However, the only valid test for image quality is to look at the display. Before buying a television it is important to spend time looking at the picture. If you like the image, then it's a good television for you. If you don't like the image, it's a bad television. Remember that you will spend many hours watching the television so make sure you enjoy watching the picture it produces. This one test is probably of more value to you than any other feature of a television.