Choose Node.js for full-stack JavaScript, real-time apps, and APIs. Choose Python for data science, ML, and rapid prototyping. Both are strong; the choice depends on what you want to build.

For I/O-bound work like handling many concurrent API requests, Node.js is generally faster due to its non-blocking event loop. For CPU-bound work, both are similar, and Python's libraries for ML and data science are far richer.

When you want JavaScript on both frontend and backend, need real-time features, are building APIs for a JS frontend, or want a lighter setup. Choose Java for large enterprise systems with strong typing and mature tooling.