One point is that the python eco-system covers a lot more ground. Ruby and the ruby eco-system basically only focuses on web development, if ruby has other strength the community is pretty quite about it.

Python is also pretty good at web development, but is also excellent at scientific computing and visualization, data processing, statistical analysis (including nice bindings to R), network servers using twisted, natural language parsing, GIS analysis, computer vision and image processing and so on. Python also has Cython which makes it trivial do use C to speed up crucial functions.

Basically I prefer python because I can use the same language for everything I need and want to do.