Assuming a team of developers are equally comfortable with writing Javascript on the server side as they are with Python & Twisted, when is Node.js going to be more appropriate than Twisted (and vice versa)?
Twisted is more mature -- it's been around for a long, long time, and has so many bells and whistles as to make your head spin (implementations of the fanciest protocols, integration of the reactor with a large variety of other event loops, and so forth).
Node.js is said to be faster (I have not measured it myself) and might perhaps be simpler to use (if you need none of the extra bells and whistles) exactly because those extras aren't there (kind of like Tornado in the Python world -- again, I have never measured relative performance).
So, I'd absolutely use Twisted if I needed any of its extra features or wanted to feel on a more solid ground by using a more mature package. If these considerations don't apply, but top performance is a key goal of the project, then I'd write a simple benchmark (but still representative of at least one or two key performance-need situations for my actual project) in Twisted, Node.js, and Tornado, and do a lot of careful measurement before I decide which way to go overall. "Extra features" (third party extensions and standard library) for Python vs server-side Javascript are also much more abundant, and that might be a key factor if any such extras are needed for the project.
Finally, if none of these issues matter to a specific application scenario, have the development team vote on relative simplicity of the three candidates (Twisted, Node.js, Tornado) in terms of simplicity and familiarity -- any of them will probably be just fine, might as well pick whatever most of the team is most comfortable with!