If you go along to your local electronics emporium, take a look at the back of the LCDs and at the back of the plasmas. I have noticed that plasma screens have a LOT of fans on the back. Reason? Greater energy dissipation. You won't hear these in a noisy warehouse full of muzak and screaming kids, but you can't guarantee you won't hear them in a quiet passage of the latest suspense thriller you buy on DVD, or during a snooker game (quiet please, ladies and gentlemen
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A few years ago, plasmas had the edge on LCDs in terms of picture quality, but looking at the latest LCD screens the picture is just as good now. Plasma TVs get a bad rap for the life of the screen, but to be honest a decent plasma TV will now last just as long as your cathode ray-tube TV: it's not really an issue anymore.
Go into your local Currys, Comet or what-have-you. Take a look at the picture on an HD-Ready screen showing a high-definition video, and compare it to the large screens showing a standard-definition picture. It will blow you away!
If I were you, the things you should be checking are:
1) Is it a high-definition screen?
and
2) Does it have a HDCP-compatible HDMI connector? (this is necessary for use with high-definition TV services: it MUST be HDCP-compatible)
Sky, Telewest and ntl all have high-definition TV services in the pipeline (already available in some areas). If you are an avid sports fan, high-definition is a definite must.
The other factors, picture quality and value-for-money, I think you would do best to see the screens "in person". It is very much personal taste, and in my experience the most expensive screen doesn't always have the best picture!
To actually answer your question though (I got to it in the end!) I would advise you to go for LCD over plasma.
Weight (I believe LCDs are lighter, therefore your mounting options are greater)
Heat production/energy efficiency (and therefore fan noise)
Picture quality (LCD is at least as good as plasma now)
Screen life (possibly)
