More or less the same thing, but I'm partial to Xine. I keep both around for different purposes though; Mplayer plays just about any file format and has a Mozilla/Netscape plugin, while Xine's a bit better with DVD's. Xine has built-in DVD menu support and de-interlacing features, which is really helpful if you play any overseas DVD's/anime. The other thing about Xine is that it actually uses (surprise surprise) the Xine libraries, leaving you open to other frontends (Totem).