Here's what I learned from a friend who's gone through the same comparo-shopping:
Plasma and LCD TVs tend to have a similar service life. However, a plasma TV will slowly degrade in picture quality and brightness every time you turn it on up to the end of its service life, when it will no longer be watchable. An LCD TV on the other hand, will maintain brightness and clarity up to the end of its service life, when it will typically have a complete screen failure. There is then a potential to have the LCD TV rescreened at a lower cost than buying a new TV.
Remember, this is third hand knowledge, so take it with as much or as little salt as your diet permits.
Mike in Michigan
"Do not kill. Do not rape. Do not steal. These are principles which every man of every faith can embrace."--Murphy McManus, The Boondock Saints