Does Python seem to have greater library support than ruby? (For example, their OWL and RDF libraries seem much more developed. Is this true in general?)
I can't answer for everything, but the library support for scientific and mathematic libraries in Python is much greater than that for Ruby. Things like matplotlib, scipy and numpy make Python almost equivalent to Matlab for these areas.
Python is, in general, a much more mature language than Ruby in my opinion, and as a result, many more developers have been encouraged others to develop mature libraries.
Ruby has recently seen a resurgence with the popularity of Rails, but a single MVC framework is a rather narrow use of a language. Conversely, Python is much more popular than Ruby for doing just about everything else, and is beginning to compete with Rails with solutions like django, as well as the Google App Engine.
Python's consistent use over the years, and its popularity in the scientific community, has spurned a lot of library development. Additionally, and perhaps most important, Python's well-designed C API has made it relatively easy to port a lot of C libraries to Python.
Python in general is more mature than ruby (ruby appeared in the Englih-speking worlds only in this century), but in terms of libraries one can only say: it depends. There are a lot of libraries that are more mature and usable on ruby, but for some other cases you should stick with Python. Generaly rubyists tend to go on one way (good example is that their YAML implementation is very good, but XML support was always left behind (weel, at least until libxml-ruby was reincarnated) ). Imho python is better for mathematical stuff, and for working with C/C++ applications while ruby is better for everyday scripting and creating simple web applications.