I've been developing websites since 1996 myself, and MDN hasn't really been on my radar for an html reference. I did use it extensively back in 2009 when I was doing work with <canvas>, and back then MDN didn't really have a general html reference. I think it's only in the last couple of years that MDN has really developed their reference documentation.
So, if you've always used MDN you really must be a young whippersnapper :)
I think MDN has an advantage in that they've only really been around since HTML5 has become standardized, whereas w3schools has had to deal with the changing standards of html4/xhtml/html5.