There is no harm in either. Overall, just have fun and learn JavaScript :)

ES5 is the current standard in all major browsers, so using its functionality could be considered "safe". Thankfully we have transpilers (will convert your ES6 code to ES5), so you can use ES6 features in production today - just remember they are converted to an ES5 implementation of said functionality.

Whether you're writing 5 or 6, it's still worth looking at and getting familiar with the various Javascript build tools (Grunt, Gulp etc). They're an increasingly important part of a Javascript developer's toolset, and using them to transpile 6 to 5 will get you off to a good start.