I would have to say that a combination of the two is always good. Culture is good, because it prevents, as Ginger Ale and Tomoyo said, flipping, and it may even flip some cities over to your side, which is always a nice surprise. However, power is more important, as it gives you control over which cities you want, and it is essential for defending yourself. As well, culture is helpful in controlling recently captured cities, which have a habit of flipping back if your culture is weak.