Zekrom is a Physical Attacker with stronger physical Defense whereas Reshiram is a Special Attacker with stronger Special Defense. The one who strikes fast will win as Zekrom would OHKO Reshiram with Outrage with 252 Attack Evs vs 0 Defense and 0 Hp Ev Reshiram. The opposite is same, Reshiram would OHKO Zekrom with Draco Meteor with 252 Sp. Attack vs 0 Sp. Defense and 0 Hp Zekrom. Therefore, both Pokemon are equal but one advantage Zekrom has over Reshiram is that it can learn Stone Edge, a physical move that is super effective against Reshiram.