Are we talking Borg before or after Endgame?
I'm certain the Borg would be the Jem'Hadar, hands down. You can't overcome the Borg with brute force; they'll just adapt. Every time they've been beaten, it's because someone outsmarted the hive mind. I doubt the Jem'Hadar could do that.
I don't think hands down, the Jem'Hadar don't mind killing them selves to deal a serious blow to their enemies. In addition to that i don't think the borg would have an easy time assimilating the Jem'Hadar either. Their body's need Ketracel-white which the borg would have to adapt, especially considering they don't have any experience dealing with white. The Jem'Hadar also use kar'takin in close quarters combat , and as we know from many episodes dealing with the Borg they cannot adapt to Melee bladed weapons. Along with the Jem'hadars ability to shroud (turn invisible) they wouldn't even see the Jem'Hadar coming. I think it would be a good match up, and if the Jem'Hadar had a changeling on their ship the borg would TOTALLY lose!
The idea that the Borg would do anything other than annihilate the Jem'Hadar is laughable at best. The Jem'Hadar's dependence on ketracel white is in no way anything other than a weakness. The Borg don't need any "experience" with ketracel white to understand it seeing as how they're the most technologically advanced beings in the known galaxy. Keep in mind that Borg nanoprobes are capable of restructuring the Jem'Hadar's genetic make up to remove this weakness all together but even if they weren't it's the Jem'Hadar who need the white to function not the Borg. Combine this with their superior numbers and weapons technology this would be another easy win for the Borg. The Jem'Hadar's ability to use bladed weapons and temporarily shroud themselves would not change that fact.
As far as starship technology if one cube was capable of destroying over two dozen Starfleet vessels in a matter of minutes do you really think Jem'Hadar attack ships woud fair any better? =
The Borg, because of their technological superiority. The Borg would simply adapt their shields to make them impervious to Dominion weapons and then just desroy or assimilate all resistance, just like they did against the Federation and all other empires that don't have super advanced technology.