It's pretty simple, we started off on Ubuntu for our linux boxes. Then made a decision to move to CentOS for the stability and vendor package compatibility for our core services. However, We are going to stick with Ubuntu on some of our back end servers - management and utility servers for the larger package repositories. Our Core Q&A engine is written in C# with MS-SQL databases so obviously we need to run windows for that.

There are only 2 sysadmins so yes we work on all OS's