[acconrad]

This is sort of like asking should you learn Swift or Objective-C, albeit much less difference between syntaxes. You can certainly get the job done on ES6, and the future is certainly in that direction, but all browsers still run ES5, so if you want to get paid for your work, you'd need to still know that syntax.

Babel is a great tool to let you use some of ES6's features now, but it will still help in debugging to know ES5 syntax since Babel transforms it at runtime.