VLC allows resizing of the video window (if you want to play a video and still browse the web with the rest of your screen real-estate) but it jitters and pixelates on occasion (while playing from a network location) where MplayerX buffers and plays smooth but does not allow for the video display area to change.
I'll have to try when I get home but I'm pretty sure both will invoke the discrete GPU (on Macs configured with one) so they may use around the same. I can use coconutBattery to see what the draw from the battery is while each app is playing a video.