Home: Ubuntu on my desktop & laptop. I also have a Mac. Virtual machine of XP when needed
Work: Windows Vista (need windows for the applications we use). I also control the servers (2 Linux, 1 Windows 2003)
I wont comment as to why I think one is better than another, only use my stock reply in such discussions:
"My choice of (OS | Programming Language | God | Sexiest Catwalk Model | Political Party | M&M Colour| Sports Team) is so much better than yours that I dont even need to justify why!"
(delete as applicable)