The lifespan of both types of displays is very similar... That is, they should last upwards of 10 years and when they fail it is as likely that an electronics component inside will go bad as it is something related to the LCD/plasma elements.
This is something I put up for your consideration:
No video games, image matters above EVERYTHING else, lots of high quality sources, bigger size matters, no large windows with lots of reflected light: A quaity plasma is the only way to go.
Possibly video games, large windows in the room, possible burn-in sources to be used, not as critical of a location, a more durable display is desired: LCD is likely the way to go.
Time after time, plasma bests lcd for image quality, but that difference is getting smaller and smaller every time the two technologies are reviewed. The strengths of LCD are few, but the flat screens that don't reflect light and their inability to have burn-in really are big plusses when you don't have 100% control over the environment the display is going into.
Always remember: The cheapest displays, from both sides, are just that: Cheap! You can get so-so quality, but for the most part, don't go overboard with your expectations if you are looking at the cheapest of the cheap for your displays, no matter which format you go with.
I personally went cheap - got a 32" Olevia on black Friday for my son's playroom. He couldn't be happier.
