There's not very much difference at all any more between the infamous "black levels" on LED or plasma.
There is still a big difference, particularly in the detail available in those blacks. Just yesterday I was standing in our showroom looking at an LED and a plasma side by side. The scene was a indoors and the character on screen had a black jacket on. Watching the LED you could see he had a black jacket on. On the plasma you could see that the jacket and embroidery and other details/highlights that simply weren't visible on the other set.
The Problem comes with out plasmas and LCD/LED's generate light to produce the image. Plasmas use a matrix of cells that when supplied with current emit colored light (typicially red, green, blue). If the scene is dark the plasma can choose which pixels aren't lit on an individual basis, no light equals black.
LCD's have a back light, if they want black they can close the individual crystal elements but the light is still there, this is why LCD's have traditionally struggled with black levels. LED technology was a huge step forward because it allowed a processor to control the back light on the fly, and began to approximate what was possible with plasma. The first LED/LCD's had the illuminating elements in line behind the LCD's, however in order to get the TV"s thin they had to move to an edge-lit system which sacrificed some performance.
