Plasma is already out of favour and a lot of the major brands no longer make plasma tvs.
It is still the case that if you want picture quality stick with the traditional CRT televisions - they still have the best picture.
If you are determined to have a flat screen tv, hang on as long as possible - prices are falling and will continue to fall. After Christmas may be a good time to buy.
Plasma tvs can suffer burn in(static image burned on to the screen which is impossible to remove )- which is impossible with lcds.
Size wise, lcds are getting bigger and they have a better viewing angle than plasma. So size is no longer the determining factor in your choice.
LCD tvs have a life span that is usually twice that of a plasma tv.
Plasma tvs are very heavy - much heavier than lcds. If you want to wall mount your flat screen tv, this must be considered.
Plasma tvs are fragile, and easily damaged.
Plasmas use twice as much electricity as lcds.
So far - lcd is favourite.
But plasma has a better refresh rate - so no 'trailing' or stuttering of the picture.
Plasma has a better colour saturation - so the picture looks better, but lcds are improving, especially in the high end market.
But as I said, you can't beat traditional CRT for picture quality. If you don't want to wall mount it, now is a good time to buy a huge CRT - the price of these is very very good at the moment.