FILTER.
Both could be dangerous, as their paths to the other would cross in the middle. Keeping it contained along the edge could be easier to manage, but I don't like the idea of using our military and our land to referee their skirmish. We shouldn't encourage the idea of Babylon as their battleground.
But I don't like picking sides, either, as this fight has nothing and should have nothing to do with us. If we did, we'd have to be certain of who would win as there would be backlash.
We'd be asking something in return at least? Toll?