The difference isn't obvious, especially since it has never been established that, if inserted into the Borg central consciousness, the geometrical shape actually would have destroyed the Borg.
But we do essentially know that in retrospect. Remember that part of the reason Picard didn't deploy the geometric paradox is because they now had a better "virus" to infect the Borg with -- individuality. We know from "Descent" that Hugh's individuality did infect his entire cube and turn its drones into individuals; in fact, when "Descent" aired, it was ambiguous whether it was just that one cube or the entire Collective that had been so transformed. But in retrospect it's become evident that it was just the one cube, and that the Collective presumably cut off contact to protect itself from the "infection." It follows that if the paradox had been deployed, its effects would've been similarly limited in scope.