While both are great and there are some web application frameworks for Python, I still see it shining a lot more in data science applications.

That being said, I recommend Node.js. You will be using the same language you’re already working with on the frontend, there’s a large community producing lots of content and tutorials, and also an endless choice of frameworks and tools to choose from.

Both are great for backend development, it really depends on what type of project you aim to build and also the team around it. Node.js Is a pretty popular choice since you can use the same language you're using in the Frontend and it lowers the learning entry barrier. Given the vast community, packages, and performance, Node.js is a solid choice for backend development. Python is also quite popular for AI and data science, it's not uncommon to find backends built with django and flask. Over time you may also feel tempted to try other languages for backend development such as GoLang, Rust, etc. Given that you're starting, Node.js will ease your entry into backend development.