If you're happy with a vanilla Civ-game, occasionally enhanced with a soupcon of spice in the GOTM, then the Mac will satiate your appetite. It looks pretty and runs fairly trouble-free.
If you want the full Civ experience, patched and enhanced with new versions and supported by the entire creative input of the fan-base, the PC appears -- from my iMac-induced tunnel vision -- to be the better way to go. Our publisher, Macsoft, has the monopoly on turn-based and RTS empire building games and they are not very supportive of this branch of strategy gaming. So, to mix metaphors, if you are into the beauty and simplicity of the quartet, go Mac; if you want the full orchestra you'll need Windoze.