My first vote would be for Mplayer and Xine since I'm more familiar with those.
-- Mplayer works fine for lots of file types, but lacks real (working) DVD navigation
Mplayer seems to be the basis for web browser plugins.
Mplayer has command line abilities to do lots of media format conversions
Mplayer seems to be focused on being an 'engine' rather than a stand-alone app,
and as such, many of its huge list of features have no (or minimal) GUI for end users.
-- Xine has more DVD playing GUI features, but seems slower even with the GUI off.
Xine supposedly has the ability to be a browser plugin, but may handle fewer
oddball file formats.
One seems to play some things the other won't (at least in current versions). So they tend to complement each other on both what they can do as well as how well they do it. At current
development stages, it seems both are needed in a good distro.
What would be ideal is to wrap some additional (and optional!) GUI interfaces around Mplayer so that it's real functionaliy could be easily accessed. With tons of devices now handling various media types, the ability to manuplate and convert between them is becoming more important. For example I've a tiny Mitsubishi video projector with an SD card slot that will play certain movie types like .avi, but, of course *not* the sort of video files my camera makes (.asf windoze-containerized). And even .avi has enough variability that compatability is problematic without conversion capability.
Since VL is supposed to be both fast, and small, and efficient, whatever the choice becomes, I'd hope that those attributes are paramount. XFCE compatability is a must since KDE and Gnome seem to be in a continual chase windoze bloat mode.
KISS should be the main rule!