I've gotta go with the Dominion.
The Dominion are close enough to Borg space that the two have likely been well aware of one another for quite a while, if not centuries. If the Borg had felt they were capable of subduing the Dominion, they would have likely already attempted it.
Wasn't Voyager just as far from the Wormhole as it was from Federation space? That indicates that the Dominion is far from the Borg.