Well, there's also the water down effect to consider. The Borg and Dominion are both guilty of this. Throughout the run, the bad guys, who are unstoppable when we first encounter them, get watered down considerably to the point where the good guys can beat them.
When we first encountered the Borg, they laughed at everything the Enterprise-D threw at it, destroyed an entire FLEET and nearly assimilated Earth with a single Cube. By the end of Voyager, Cubes were largely ineffective against Voyager, even the ramped up tactical cubes had to shut down for repairs against a ship half the size of the E-D to say nothing of them turning on Godmode in Endgame and blowing up Cubes wholesale.
When we first encountered the Dominion their technology was unstoppable, their warriors were the best, and they took out a Galaxy class starship with little effort. By the end of Season 7 of DS9 runabouts could take out Jem'Hadar attack ships.
So as for the hypothetical Borg vs Dominion? Really depends on when we're talking about. I'd say early DS9 the Borg would win. By the end of Voyager the Dominion would by sole virtue of the Borg having been watered down more than they were.