Ok, I'll try and keep this quick and informative, without boring you with all the technological jargon. There are advantages and disadvantages to each. While they are both flat-panel displays, LCD's tend to be lighter and brighter and are not susceptible to burn in, while plasmas have been the leader when it comes to contrast ratios, color fidelity (though that is subjective, and depends on the viewing area) and refresh rates. The gap between each technology's pros and cons lessens as time goes on, however, so your decision making process was not as easy as it once was. In the past it was a matter of asking yourself a few questions. Want a larger display? Plasma. Want a brighter image? LCD. Want richer blacks? Plasma. Want a display to double as a PC monitor? LCD.
Quite honestly, the consumer is in a harder place when it comes to buying a flat-panel display, as things are not cut and dry anymore. If you're dead set on buying a flat-panel, than I'd say go down to your local electronics store and compare for yourself, side-by-side. My advice, however, is to stay away from flat-panels altogether. There are displays out there that are cheaper, have arguably better image quality, and are almost, but not quite, thin enough to be wall-mountable. Sony's SXRD (LCoS) line of displays are fantastic; Samsung's and Mitsubishi's range of DLP's is also a great choice; and JVC's HD-ILA displays are equally as impressive.
I'd personally recommend Samsung's (DLP) HL-S5688W
BestBuyPlasma.com | SAMSUNG HL-S5688W 56-IN Widescreen DLP HDTV with 1080p Resolution
or, the display I own myself: The Sony (SXRD) KDS60A2000
BestBuyPlasma.com | SONY KDS-60A2000 60-IN Grand WEGA SXRD Rear Projection HDTV
Both have much richer blacks than most flat-panel displays and both look fantastic in any home theater environment.
Hope that helps... a bit.