It all depends on your viewing habits. LED is generally better for video games, computer and brightly lit rooms. Plasma is better for most movies and darker rooms.
Of course there are many other things to consider and there are exceptions to the above conditions, depends on the TV itself... but one is not better or worse in all situations.
plasma is better for video games because the screen response time is always killer. better for brightly lit rooms because of superior contrast and black level performance, and dark rooms for the same reason. lcd is better for... your wallet. and power consumption.