The Dominion wins, hands down. It's no contest. Why? The Jem'Hadar and Vorta are genetically engineered so that they cannot be assimilated. Biologically, anything that enters their bloodstream is destroyed-including nanoprobes. It states specifically they are immune to poison on screen. This is the same situation as species 8472. What the Borg cannot assimilate they cannot understand or adapt to. Therefore it becomes a battle of conventional weapons in which case the Dominion pose much more of a threat then the Federation ever did. They clone at an alarming rate, the ram ships like its nothing, and their Phased Polaron Beams penetrate shields unless you come up with a countermeasure. The Borg cannot do that with no knowledge from assimilation. As powerful as the Borg are, they have their matches. Anything they cannot assimilate is bad news for them.