Civ3 vanilla version 1.29b2 for Mac is pretty stable in my experience. I can't compare it with the PC version as I don't have it, but the Mac version works fine for me.
The PC has the two expansion kits as well, which are not currently available for the Mac, and the editor for the Mac version is not great, so if you are interested in modding you may prefer the PC version. It only supports version 1.21, so you can't place player start positions, for example.
A Mac is just a whole better experience than Windows in my opinion, and even if I could play no games at all on it I would still use a Mac rather than a PC. If I just wanted a games machine I'd probably have a PC ... or a PS2.
[EDIT] By the way, I voted Civ3 for PC is better, simply because there are more options - PtW, Conquests, Editor.