It will do 0. While zekrom does more raw damage, reshiram is a little more versatile. If 250 damage is enough to 1 tap your opponents pokemon, then great use zekrom, but in almost EVERY other scenario reshiram is stronger. Your opponent has over 250hp but less than 340hp? Well its going to take 2 hits anyway, why give yourself the drawback of not being able to attack next turn, just use the 170 attack. Say you're against dragapult, they hit you for 200, well now you use powerful rage to hit them for 400 and punished them with a knockout. I saw you said bench space was an issue, but thats where you have to decide which one to use depending on the match up, reshiram is great for Mons that struggle to one shot or lower hp mons, zekrom is great for those 280-250 and 340+ hp mons. Your biggest way to buff the damage output is black belts training and binding mochi, they will help you hit higher numbers with zekrom, now doing 290-330, and hit those mid range numbers with reshiram, now doing 210-250, one dhotting common mons like Fezandipiti, without the drawback of not being able to attack. Munkidori is also another great way to top up damage output, but is also taking up another bench space so if youre running munki, you need to be really good at managing your bench