Neither, since neither had an OSX port at release.
Aren't we through yet with the "wah Civ 5 sucks!" stuff yet?
Anyway, for those voting, consider that Civ 4 has had many years to make refinements to the base game. Civ 5 hasn't even been out a month yet. I know the OP said that they should both be considered to be bug-free, but I honestly doubt y'all are capable of removing that bias from your minds. I know I'm not. Most of you have hundreds of hours invested in Civ 4; of course you're going to like it more than Civ 5. Hell, even if Civ 5 didn't have bugs and bad AI, you'd be complaining because it's different, therefore it sucks.
I'm not saying you're stupid, just that you're human. Most people don't take change well, especially when they've invested a lot in the way things used to work.