I would go with Luthor.
Constantine is a badass, yes, but a solo artist. When he involves his allies, they get hurt.
And his allies know this.
Getting them to help him would take some doing, and is not impossible. But anyone who is anyone in the magic community will know of his rep, and will be hesistant to help him.
Luthor, on the other hand, does not have this rep yet of being a bastard. He could convince others that he's a good guy, and will likely have a larger group to help him as a result.
Of course, John would still win a war lol. But will have to do so with fewer allies than Luthor.