The Dominion. They're more flexible and they can think.
Just taking one example, their policy towards Bajor. Their Cardie allies assumed that the Dominion's assurances of Bajoran neutrality were a fraud that would be rescinded as soon as they got control, but Weyoun laid that notion to rest.
The Dominion is smart enough to understand that the good example they set with Bajor is far more valuable than the greater efficiency of imposing oppressive rule. The Borg are completely incapable of clever, nuanced thinking like that.