I use and like Fedora, mainly because at work we use Red Hat servers and the two are pretty close, so I know my code will most likely run without any modifications. Overall, there's not a lot of difference in the more up-to-date Linuxes. I prefer Fedora's package management (yum) over eg. apt-get, but that's really because I'm used to yum.
The best thing to do is try out some distros. A lot of them have live CDs, or you can do a full install in a VM using either VMWare Fusion's public beta or Parallels trial, if you don't already own Parallels. I found that Fusion supports a wider range of *nix installs - for instance I couldn't get Ubuntu 7 or Solaris 10 installed in Parallels, but Fusion worked fine (other people haven't had any problems with Solaris on Parallels)