The debate that won't end. This topic come up every three weeks or so it seems.
Look the Borg are on paper superior but the Dominion is so vast and powerful it would give the Borg a very tough fight. On paper the Borg should win although if the Borg can't stop Voyager maybe I shouldn't be so quick to make that prediction.
That being said I care less about who is more powerful and more invested in which group is more interesting. After Best of Both Worlds Part II the Borg were so badly mishandled and waterdowned that they became a boring species (too powerful for inept writers to handle properly in the later years), a shadow of themselves. The Dominion, with its caste system, its political manipulation and its need for order, became the more well developed and better handled of the two. The Dominion also came a lot closer to conquering the AQ then the Borg ever did. I would rather write stories about the Dominion any day. After "Q Who" and "The Best of Both Worlds" (particulary part one), the Borg started to slide as antagonists. And, yeah, I'm including the overrate film "First Contact" with its ridiculous plotholes and absurd actions by all the characters involved. And the introduction of the Borg Queen? Unnecessary.