In terms of sheer physical force, the two are about even. I think it would depend on whether the Borg could assimilate the Dominion shape shifters and their Jem'Hadar warriors.
If so, yes, the Borg would win, and assimilate the Dominion.
But if not, I think the Dominion could eventually eliminate Borg resources, like they did to Federation and other species in our quadrant.
Perhaps the most disturbing possibility, is that the Dominion might find a way to incorporate Borg nanotech into their own selves without being assimilated, resilting in a superior Dominion/Borg hybrid. The hybrid would encompass the best features of both species, as well as having both their resources. And since the Dominion actually have a plan for conquest in place, where the Borg expand their empire mostly by instinct and brute force, The hybrid would actually have a pretty good chance of overwhelming the entire galaxy.
Unless the Q shuts them down.