Well, twisted is an async framework, and that's the important thing. Back in the day I actually wrote a Twisted protocol to send/receive SMS messages from a modem over serial. In total the code was like 100 LOC. That's how flexible, well-organized it is.
~ Mature. Twisted has been around for a lot longer than Tornado and has learned from all of the available history of modern OS networking.
~ Tested. Twisted's been in production for a long, long time, and is covered with thousands of unit tests. It's official policy that code may not enter the Twisted tree without accompanying tests.
~ Flexible. Twisted can be used as a general-purpose networking library, it can integrate with Pygame and Pyglet, GTK+, Wx, Qt, Tk; it doesn't have to be used for servers.
~ Extensible. Twisted's connectors are explicit, and rely on interfaces and adapters rather than inheritance. As an example, Twisted's SSH library lets you separate the SSH server, SSH channel, and SSH shell from each other. Annoying if you want a standard SSH server, but terrific if you're building a custom SSH proxy or tunnel. (I did this a few weeks ago at work. A lot easier with Twisted than with Paramiko!)
I should note that it's not an either-or; there is a branch of Tornado which throws out all of the event loop and uses Twisted's event loop instead.
That branch/port of Tornado is https://github.com/fiorix/cyclone and it's pretty nice. I greatly prefer it to t.web or nevow, which I used to use.
The tornado and cyclone APIs are closer to what I am thinking about as a web developer. Virtually never do I make an object and then think about how I'd like to adapt it to return an HTML view or programmatically add it under a parent object.
