Plasmas generally have a better picture since they use individual gas elements rather than LCD screens which use shutters to block RGB light. In addition plasmas also have wider viewing angles, richer black levels and faster response rates.
That said, I would no longer buy a plasma TV since LCDs (especially LED LCDs) are getting to the point where it's hard for the average person to tell the difference. Also, plasma TVs use A LOT more energy (e.g. 600W vs. 100W) and suffer from burn-in over long periods of time.
So, while plasmas have a slight picture advantage, I'd go with an LCD these days (Samsung has some good precision-dimming LED LCD TVs).
If I were to buy a new TV today, I'd go with LED over LCD or Plasma, but it all depends on what's most important to you as a viewer. It also helps to consider your home entertainment package as a whole. As with all things electronic, your system is only as good as your weakest link. So it makes no sense to bust the bank on that LED if it means you can't afford a Blu-Ray player or decent HDMI cables. But if price isn't an option, Plasma and LED are the clear winners. However, as someone already mentioned, it really depends on personal preference. Just remember that if you go to BestBuy or Fry's to check out the inventory, different settings of the monitors can make them tough to compare.
It depends in the room you are going to put it in. We have one of each. In the "Man Cave" there is only one window so there is minimal glare and the plasma works great in there. However in our family room there is a whole bank of windows with lots of light and the LCD is better for in there. We had talked to several different people in several different stores and they all said the same thing when we started asking the right questions.