So the Cardassians are more dangerous if you are compliant, and the Dominion is more dangerous if you are not ? What a comfortable choice this is.
I'll stick with the Federation, thanks.
Under the Dominion, you would be oppressed. Under the Cardassians, you would be enslaved and oppressed.(Slavery and oppression are not the same thing.) It's a huge difference, until you consider that the Dominion would seek out government's like Cardassia's to police the Alpha Quadrant in the Founders' interests.
For the sake of argument, what would have been the effect if the Federation, Klingons or the Romulans had decided to join the Dominion? I suspect that the Founders would have been satisfied so long a order was maintained. However, it would be left to the collaborating governments to maintain order with the methods they have at hand. Those societies might only become more repressive by a matter of degrees; there might be no substantial change. Romulans, who are accustomed to be spied on by the state, might feel little change. The Klingon warrior class might feel empowered. However, Weyoun specifically picked Earth as a center of future resistance and recommended eradicating the population. What does that mean? Perhaps he would have no faith in the Federation's ability to control th population.