Ubuntu (gnome desktop) or Kubuntu (KDE ) if you want easy to get it  
going, these are definitely easy to install (and based on Debian).  
After getting them going, though, Ubuntu has a lot less customization  
you can do. Kubuntu leaves you with more control over the GUI because  
its KDE. You can set it up to be more Windows like, more OS X like or  
something else all together.

If you want a lot of flexibility, Gentoo.

If you want to match your servers, go with Fedora or Red Hat, Suse or  
OpenSuse, or Debian. They're all Linux, but pretty different from  
eachother in surprising little ways.

It's a tough call. Grab a few older machines, and try installing them  
and messing with them.

I've even tried YDL (yellow dog linux) for power pc computers  
(only?). And it has a nice installer, but a terrible interface called  
E17.

But while you're at it, if you're planning to do Rails, seriously  
consider OS X as well, you can run Windows on the same machine these  
days, so you have a one stop dev shop, and OS X is a BSD with a nice  
Bash. It's pretty popular in the Rails community, and for good  
reason. It's got a lot of good Rails development tools available, and  
workflows well established.