The salesman as usual is talking complete bull when he claims the TV screen is tougher.
It is not necessarily true that either technology is tougher. When the Wii controller hits the screen no LCD or plasma will survive. Unless it's one of the Panasonics with the front glass plate. That will break, but usually the plasma display inside will survive. Still an expensive glass plate to find and replace...
I guess they're scared of the guy getting electrocuted when he cracks the screen as it is off when they do that. However, there is limited shock hazard. When the screen cracks, the electrodes in the panel short, and the TV makes a rather horrid buzzing sound :/. You lose gases and typically picture goes immediately, but sometimes it fades slowly over about 30 seconds.