For learning, in my opinion you should pick the language that you feel most comfortable with, and then roll with it.
Every framework has its advantages and disadvantages. While you're learning, these things shouldn't matter, rather learning the concepts should be more important. In order to learn how the back end works, you should just make something. It doesn't have to be amazing or complex, but pick a small goal for the project. Maybe something like "I want to learn how x works on the back end", or "I want to implement OAuth in this project", and then just start doing.
As for understanding Node better, learning Node frameworks/libraries would be best, but if its the concepts behind the back end development that confuse you, I think learning something you're comfortable with first would definitely help. :)