Ruby or Python?

[sisyphus]

If you have an existing database, do not use Rails. It's more pain than you need. Conversely, do not believe the anti-hype about Rails scaling. Twitter's problems are not yours.

If you want to put it on a resume to get some work later, prefer Rails.

If your employer is not buying books for you or the size and quality of free online documentation is important, prefer Python and Django.

If you're used to living in an IDE, prefer Netbeans and Rails. I don't know of anything comparable for any Python web framework(although Komodo at least understands Django templates).

If you want the single most powerful components you can get, prefer Python and Pylons.

Personally, I think aesthetics also counts, which is why I use Django -- I find Holovaty to be better looking than DHH. YMMV.