I've been using both for weird format files and codecs (anything more or less standard is streamed to the tv directly) and felt that VLC tends to try as much as possible to read everything, while MPlayerX is more tolerant towards garbage in the stream and won't bother recovering mildly invalid data.

Part of the lag in VLC when starting a stream might come from a more heavy handed init process.

This can be a curse or a blessing, I'm happy both approaches are available.