I have both VLC and MPlayerX, and use both.
MPlayerX doesn't have a playlist feature. So, you double-click a movie file on disk and it loads up and plays. When the movie finishes, it provides a next button which can lookup the next sequentially numbered file on disk. So, after watching one episode of a TV show, hit next, and it will pick up next episode of the same TV show. MPlayerX feels lightweight, great for one movie at a time.
VLC has a playlist feature. Every file you add goes into the playlist. It's great for dragging in a folder(s) of content to watch at a time, such as a video tutorial, and it shows the time/duration of everything on the playlist. So you know how much time to budget to finish the set of videos. It also has a "sticky playback rate", which means you can speed up playback to 1.7x for video tutorials and it will remember that setting while it plays through the playlist. It also has convenient gestures such as using scroll wheel up/down to change volume. Further, it provides an equalizer which you can set to "Live" or "Headphones" preset to add some clarity to video tutorials while playing them back at 1.7x.
Both are great apps.