Plasma is better for larger screens, like 50" and above at the moment. LCD screens really dominate the 30" market and below, however that's rapidly changing with how much LCD technology is catching up. LCD's have a thing when dealing with color, specifically blacks. It's extremely hard to get a "true" black on a LCD. I don't know if it's much easier on a Plasma, IMO, but the colors on my Vizio look brighter and richer than the colors on my mother's LCD.
Plasma works better for darker rooms, although my 52" is in an extremely brightly lit room where the sun often shines right through on the screen from time to time, and it honestly doesn't affect the picture all that much, IMHO. I know when it comes to LCD monitors for computers, ghosting is an issue for gamers, where the crystals don't update or refresh as fast enough rate and the image leaves trails in places. I'm assuming the same goes for LCD TV's as well, however with High-Def resolutions and action movies and the like, I think it's safe to assume that most LCD's have extremely minimal ghosting, if any. At least the top of the line ones shouldn't.
Honestly though, the quality of the TV picture depends on the TV itself, the quality of the signal and what you're watching. If you try to watch "normal" TV on a high-def TV, the picture is going to look like utter and complete shit unless you have an upconverter box, and even then the image can still turn out distorted, blurred and grainy. If you have cable or satellite (I have DirecTV and it's fucking amazing) and you have a cable box or satellite receiver, if it's got an HDMI connector, using that for your TV input as opposed to a coax cable or S-Video or The colored cables (Yellow,White,Red or Blue,Green,Yellow or whatever) will give the BEST picture.
TV brand is extremely important, as is cost. Cheaper doesn't mean the same quality at all, nor will the picture be the same across all TV's because of a cable provider. My aunt and uncle have a 42" Panasonic LCD screen, and they've got Comcast, and they've got the HD sports package. On their TV, watching football makes me cringe. The image is bright and crisp and vibrant during low-movement scenes, but once players start running the image becomes very grainy and gritty (one sign of ghosting).
Basically, when you go to buy your TV, look at the display models. You'll be able to visibly tell the difference in brightness, picture clarity and quality, vibrancy, contrast...etc. etc. LCD/Plasma....either or. Both of them will loose 1/2 their brightness over the course of their product life (Unless you have a very expensive LCD with LED back lighting as opposed to fluorescent back lighting, but LED-LCD's are still fairly new and rare from what I can tell. They will become commonplace faily quickly though, thus making LCD the far superior in terms of product life and durability.)