Energy isn't the only way to understand killing power. Compare a 223 to a 45-70 in comercial loads, they behave differently in large game. The abilty to penetrate deeply enough to damage internal organs has much to do with the momentum and bullet construction. A similar situation with pistol bullets. bullet shape has some bearing also. A RN lead bullet doesn't have much shock effect compared to a SWC Keith type, but the energy may be identical. At longer range, your 38 bullet fired from a 2" barrel may not have much paper energy, but if it has a decent wide, sharp edged flat point, and penetrates enough, perhaps even all the way through your target, the paper energy isn't an issue.
Perhaps we didn't all answer the question exactly as asked, but few of us have scoped 2" revolvers. We also enjoy the discussion.
Part of the answer (in the practical sense)is how far can one effectively use the gun, and how far is the load able to kill well. The load will be capable of inflicting fatal wounds to farther than most can shoot the guns well enough to hit with them. If the charts show energy of 53 FP at 750 yards, (equal to the 22 short), it would liklely be a fatal would, as a 22 short can certainly be fatal. The 38 bullet will have more momentum, being heavier, and will make a larger hole. Doing some shooting tests on game animals with loads of various velocities would help answer the question.