If your main concern is power usage get an LCD, if your top priority is picture quality then go with the plasma. No LCD will give you the PQ, black levels, color accuracy or viewing angle of a good plasma. Viewing angles on LCDs suck. The LCD manufacture’s specs will tell you can watch the LCD from 140-176 degrees off center, that is BS. One consideration though concerning the plasma, how bright is the room you are going to have the TV in? LCDs can be turned up to torch mode and overcome high ambient light, plasma do not do as well under those situations. Normal day time light is not a problem, but if the TV is in a room with massive large windows opposite it then you may want to go with the LCD.
For the record I have a 50 inch Pioneer Elite. They are no longer being made which is too bad because they have simply the best PQ period. My wife, who is not a big A/V person, actually went to pick it out at BB. She was first drawn to the Sony Bravia, but after seeing the Pioneer she actually called an said it was the only one she would buy, it was that much better. The Panny are good TVs. I have a 10 year old 47 inch RPTV in the basement. It has never had a problem. Good luck in your search.
Lastly whatever you buy have it professionally calibrated. It cost a little money, about $300-400, but it is worth every penny. Also the comment on the cables is dead on. The big expensive HDMI cables are not worth the extra cost. Just buy some good quality cable for a reasonable price and you will be very happy.
As such, I'm still leaning towards the Sony KDL-40EX500 as my first choice, though I'm going to Fry's at their special darkened screening rooms and see if I can find a better model.