I recently did a large amount of reading on this matter. My TV of 10 years was going, and I wanted to get 3D smart tv. I debated Plasma vs. LED, and ultimately did not make my choice until I saw them in the store.
What CC said is true. Plasma *used* to have a large problem with image retention. It could ruin your picture if you didn't burn in the tv properly. Even though that has been improved, you can still get image retention, but it won't last as long. It is still recommended that you break/burn in your plasma for 150-200 hours, to make sure you have no issues. You can do that by getting burn images, which is just hours of a color slide show, or by watching regular tv that does not contain letter box or static images. That is a little tricky, considering that most networks leave their annoying logo on the screen 24-7 now. You are also asked to avoid playing video games, again especially ones with static images like a health bar, etc, for the first 200. You basically just need to turn your tv on at night and let it go, for a week.
As CC said, plasmas have glass screens and they are not as good in bright rooms. They are made for more dark locations. They also run hotter, and are not as energy efficient. When you have the tv calibrated properly (which you can have professionally done by the store you buy it from (sometimes for free) or look up the settings yourself provided by some videophile guru's. it's breeze), the picture is hands down the best. They have the truest blacks, and most realistic picture, BUT LED is just as good imo. I ended up going with the Samsung LED, 3D, Smart TV. I love it!! I didn't have to worry about breaking it in, I could go straight to my video games, and watch movies letter box if need be.
LED can still have it's own issues. You'll get flash lighting in the 4 corners with some, but mine has been perfect. I had been planning on going big or going home. Lol. Glad I did.