I would buy the Pioneer in a second over the Samsung. Pioneer, in my experience, consistently provides better image qualilty, black levels, contrast, colors, and image processing across the board, then combines it with basic engineering that is simply 'better' than the competition.
1080p acceptance: It means that you can feed the TV a 1080p source and it will convert it to the native resolution of the display. You should mostly disregard when TVs are listed as 1080p or 720p - what you want to know, and what matters, is the actual resolution of the display. Is it 1024x768, 854x480, 1280x720, 1365x768, or 1920x1080? It most LIKELY is 1024x768 on a 42" HD plasma.
Then you want to know what formats it accepts. Most often it is 480i, 480p, 720p, and 1080i. Some newer displays also accept 1080p. But, unless the TV itself is 1080p, there is often not a huge advantage for a display to accept this resolution.
Another way to put things: If you speak English, and only English then that's your 'native resolution' (language). We'll say that you have no ability to ever speak any other language - ever. Okay, great. Now, let's list the languages you understand: English (obviously), Spanish, French, Italian... and (maybe) Chinese. Those are your accepted (resolutions) languages. But, before you can say what you are hearing you must first convert them all to English.
That's the way TVs work. They have to convert from what they understand, to what they can produce. In the above example, the best conversion happens when you hear English and can speak it again as English. With video, it is the same when you feed a display it's native resolution, it tends to reproduce the best possible image. Video, fortunately, is fairly forgiving with different resolutions as long as there are good video processors in place.