Between those two handguns, a 357 revolver and a 40 Shield, the OP has to shoot one of them "better".
Have you used a shot timer and a target smaller than a silhouette to compare?
"I feel very comfortable with either one of them". <---- But you likely shoot one "better".
I have a Glock 20SF (10mm) and a Glock 17 - want to guess which I am quicker with given the same accuracy standard; well, with a shot timer I don't have to guess and all it does is confirm (put a number) to what I can subjectively discern anyway. If you guessed the 17 you are correct.
Between the 357 revolver or 40 Shield which would you prefer in hand if you had to unexpectedly defend yourself (wherever) and maybe more than one human attacker? Carry that.