If you want to do machine learning, then definitely Python (out of those two). (Asked last week).
Python is easy to learn in general, but since you already know Javascript, Node is going to be faster to learn for you.
It depends what you want to do. For website backends, both are powerful, have frameworks and can do similar things. In fact, I'd say you don't really need to learn both for practical purposes.
Personally I think Python is a much more elegant language than Javascript, but that's personal, other people are wrong think differently.