I like to use both. Sometimes MDN gives an excellent technical description of something, but W3Schools tends to give a bit more examples, and lets me play with the code.
Once I know how it works, I can look at the technical description and fill in any gaps that W3S had.
Comments
Please try to support free and open resources like MDN (https://developer.mozilla.org/en-US/) and http://www.webplatform.org instead of w3schools
I like to use both. Sometimes MDN gives an excellent technical description of something, but W3Schools tends to give a bit more examples, and lets me play with the code.
Once I know how it works, I can look at the technical description and fill in any gaps that W3S had.