I like both Python and JavaScript and I think both are very good languages that will get the job of web development done. I have a slight preference for python (used to be wider before es2015), though, and I think it encourages maintainability and readability a little better than js.
That's about as much as I can say regarding backend work. Now, for other pastures, Python has better prospects in the market. One would be hard-pressed to find JS in something other than web applications, while Python can be used for machine learning, data science, embedded systems, desktop development, etc.
But you already know JavaScript and, if you feel like web development is enough, there's really not compelling reason for stopping its use.