Ask HN: Web Programming Platforms

I'm starting a new web project soon and I'd like to try a new platform. I'm currently using CakePHP which is OK, but I want to learn Python or Ruby. With Python, I've heard the major ones are Django and Pylons, and w/Ruby, Rails of course. (Note that I don't know either Python or Ruby, but I figure I'll pick them up as I go along).

I've done some reading and it seems that these platforms have many of the same features. I don't want to start a platform or language war. I'm just interested in hearing some pros/cons of each. If I've left out a platform you especially like, please mention that also.