Counts first answer is way off the mark (sorry count)
LED/LCD and plasmas are nothing alike at all and create the picture in totally different ways. Both LED and LCD are essentially the same in that the screen works by blocking light created behind the screen, the difference being with LCD screens the light is provided by 1 or 2 large tubes whereas in LED tv's the light is created by, well, LEDs obviously. Plasma screens work by each pixel creating it's own (ultraviolet) light which in turn causes a coating of phosphors to glow which creates the picture you see.
I think, personally, at 40inch would go for a LED if money no object, or if not a LCD screen over a plasma, most of the disadvantages of LED or LCD screens have been ironed out these days but plasma screens still have the main disadvantages they always have (expensive to run, run quite hot, heavy, and worse of all, can suffer from screen burn in)
(for the record I have a samsung plasma and although generally happy with it do often wish I could have got a LCD screen at the size I have.)