I say the Borg would have the upper hand for awhile. The only problem is that I don't believe the Founders can be assimilated, although the Jem'Hadar can. The Borg would have a major problem figuring out the Dominion's secrets if they can't assimilate a Founder, since the Vorta only know so much.
However, Jem'Hadar Borg drones would be UNSTOPPABLE. Although I doubt the Borg would use their shrouding ability, since they have never used cloaking devices. There is also the possibility that the Dominion could eventually modify future Jem'Hadar warriors to be resistant to assimilation.
Both sides have the same ability to easily replenish their supply of soldiers, with the Borg just needing to assimilate some race along the way, and the Dominion just needing to plug in the 'ol cloner.
One thing that might backfire on the Dominion, however, would be if the Borg captured a Dominion cloning facility. That would potentially give them a big advantage (see Star Trek Armada).