As Rich Bradshaw points out Node.js is written in JS, which means you can keep the front end and the back end in the same language and possibly share some codebase. To me that is a huge potential benefit of Node.js.
Node also comes with more asynchronous libraries out of the box it seems.
V8 should make JS faster than Python at least that's what benchmarks seem to suggest, but it may not matter much, because both Node.js and Tornado (and most other web frameworks for that matter) use wrappers for native libraries. A lot of the Python standard library is written in C or can be replaced by a faster alternative, which mitigates potential differences even more.
Web services are usually I/O bound, so that means we're spending the time waiting for the data store and not processing the data. That makes the synthetic speed difference between JS and Python irrelevant in many applications.
