What are pros and cons for MacPorts, Fink and Homebrew?

I'm just migrating from Ubuntu Linux to Mac, and everything is new and I'm re-learning a lot of stuff. On Linux I had the excellent apt-get to manage software packages. I googled for an alternative on Mac and found about MacPorts, Fink and Homebrew. I will use this computer primarily to develop Ruby on Rails applications. So, what are the differences between them? Which are the upsides and downsides? Which one is best maintained and has more packages?