OS? Honestly both Windows as well as Linux are cool.
Choose the OS according to your preferences (Maintainence, usability)
Win+CGMiner, Linux+CGMiner, etc can be good combo.
I've mined for Gentoo/Ubuntu/Win7/Win8, and
it's more about the stability of the miner program or drivers than the OS.
Though I don’t mine anymore.
Hope that helped.