It depends. If the movie (or TV show) was filmed and shown in widescreen, then widescreen.
If it was filmed full screen, then cropped to widescreen, you will see more in the fullscreen, then the "cropped" widescreen. You get extra picture at the top and bottom that wasn't included in the widescreen.
Over all, I prefer widescreen.
And there's a special place in heck for the guy who thought up "Pan and Scan".