For example, Entei has a ATK of 235 and can learn fire fang and fire blast. Salamence has a ATK of 277 (42 higher than Entei) and also can learn fire fang and fire blast.
When against a pokemon that is weak to fire, which of them has a higher DPS? And how much ATK is enough to even out STAB?
That's correct. There's more to it when you compare different moves, as they can have different cooldown, energy use, even the damage window matters for tight situations for fainting or fighting the clock... sims work best for that.
That said, Mewtwo candy used to be very scarce, so it was best invested in Shadow Ball variants because it's more versatile and pretty much matches other moves in favourable conditions (weather, STAB, SE, etc) in terms of DPS and especially TDO.