The main advantage of node.js is that all its libraries are async so you don't have to worry much about blocking. There are async libraries for mysql, postgres, redis, etc. All is async by default.
Python have a library for anything - but most of these libraries are not asynchronous. In order to take advantage of tornado (and not to block the process) special libraries for are necessary (e.g. you can't just 'pip install redis' and use it, you'll need something like brukva), and there are much less tornado libraries than node.js libraries. There is no async mysql tornado driver available at the moment, for example (or at least I'm not aware of it).
But you can still use many python libraries with tornado (ones that doesn't do i/o), and tornado community is raising and filling the gaps.
It is easier to write an app using node.js than using tornado in my experience. I personally switched to tornado from node.js because it fits into existing infrastructure of my python project better (integration between django site serving html pages and tornado server providing realtime features was quite painless).
