Both methods help a team equally IF the WAR amount is equal. The "Why" of this is what seems to be confusing.
Teams must pitch all the innings they pitch regardless of how well any individual pitches his innings. If a team plays a full game and the starter throws 5 innings of perfect baseball and leaves, the team STILL has to pitch at least 4 more innings. There is no bonus for pitching perfect baseball.
1) Every pitcher who is above replacement level helps his team win games but he ONLY does that while he pitches. He doesn't help the team by sitting on the bench, drinking beer in the locker room or napping in the bullpen.
2) Every inning that is NOT pitched by a pitcher above replacement level NEEDS to be pitched by a pitcher at or below replacement level.
If one were to assume that there are 140+ ERA pitchers sitting in AAA ball or not getting contracts, then would be erroneous. The 'leftover' pitchers not being used are close to or below replacement level. As a result, you can see from 1) and 2) above, that a pitcher above replacement level helps his team every time he pitches because he is better than anything else they have.
The more a pitcher is above replacement level, the more it helps.
The more a pitcher above replacement level pitches, the more it helps.