Plasmas are subject to burn-in, meaning, if you leave a static image displayed for a long time (say, a menu, or a health display in a game) it can get "etched" into the display.
LCDs have a slower response time, which can lead to ghosting or smearing in games or movies. They're also usually smaller than plasmas. But they don't have burn-in.
For movies and DVDs it is hard to beat a 42" or 50" plasma. For gaming & use a monitor, I'd probably go with the smaller LCD.
I can't remember if I read that the 30" LCD requires a dual-link DVI connection...?
I was going to say make sure you get something with an HDMI input, so it will be compatible with high-def, but it looks like the industry is already preparing the successor to the the HDMI connector, so it might be obsolete this time next year.
