When I was a noob, w3schools was far more useful. I learned HTML and CSS on w3schools. Now I'm more of an expert, I need the detailed documentation that MDN has. Each site has its niche. Still sometimes reference w3schools because it's easier to read, though.
I find w3schools a better reference for quick HTML element and CSS rule lookup, but I always go to MDN for Javascript stuff, especially the newer API's that came with HTML5.