Not to diminish the contribution from shabda - but what would really interest me is hearing about these frameworks from the perspective of someone who has launched and maintained a complex system, not a toy app.
I've built my startup entirely in Python + Pylons (1.0). Pylons is wonderful, I really like it a lot and I'm excited about its future too (move to Pyramid which is basically repoze.bfg).
The application I'm running is large, complex, and has quite a few moving parts.
I run it under mod_wsgi in Apache and use the Paster server when developing on my local machine. I particularly love the unit testing harness for Nose that Pylons provides.
I think you'll find that all of them are very similar in terms of ORM, Session, Cookies, etc -- they are, more or less, the same toolbox.
However, the devil is in the details. Once you start managing something sufficiently complex you start to see what details constrain you in bad ways and such. I'd guess that you don't get to that point until way after you launch.
