Always go with W I D E screen. Let me tell you why... Full screen usually uses Pan-and-Scan. For instance, in the film Jaws, the shark can be seen approaching for several seconds more in the widescreen version than in the pan and scan version. You don't but a 50" Plasma TV to watch a standard picture.
I know this is kind of late but I just wanted to add a small comment. Under normal circumstances I would agree with everyone in buying the widescreen version, but because of the different aspect ratios the film was shot in one is maintained. The one I speak of is IMAX which has a ratio closer to 4:3. So the IMAX scenes are correctly maintained while the rest of the movie is PAN and SCAN. If you buy the widescreen the IMAX scenes will be chopped just like in non-IMAX theatres. I guess you have to choose the lesser of to evils. Whatever you may think that will be.
A widescreen TV has the aspect ratio of 1.78:1 (16:9) while a theatrical film is usually either 1.85:1 or 2.35:1 (better known as "Cinemascope"). The first will almost completely fill the TV screen (usually you won't notice any bars because of the overscan) while a Cinemascope film of course leave black bars. Reformating this to 1.78:1 would be pan&scan all over again. I've seen this done on TV actually. No, always maintain the original aspect ratio - the viewer can always distort the image anyway he/she wants by zooming or cropping the image.
Buy VHS! This is a very dumb question!. You bought a 50" Plasma HDTV, and you want to know whether to buy Widescreen or Full Screen movies? If I were this unknowledgeable about HDTV, I wouldn't have purchased one.