The downside (or what is thought to be the downside) is image retention. What that means is because Plasma is really a gas, the fear is if you leave some image on your sceen for to long it can "burn" in, or the plasma stays that color (usually its just a hint of that color).

With that said, you should take the time to break in your plasma (google it). this helps solve that problem. But its not that big of deal anymore. Most newer plasmas are built so they don't get image burn in.

LCD is just more popular and easy over all (even takes up less power). Plasma is actually cheaper to make and so it is cheaper.

I would get the Plasma, it has beter viewing angles! You won't regret it! I have a 42 in Panasonic Plasma and love it for xbox 360