Both
We've got a pair of both and like them all. The .38 Super is a nice round to shoot loaded heavy, light, slow or fast. Brass is not common. Factory loads from the big boys are pretty weak so some fool does not blow up his .38 Auto with a .38 Super Auto load. So you've got to handload to have any fun with it.
The .40S&W is as common as dirt. Seems like all the cops are shooting them I have guys at my club giving away .40 brass to me. This round can also be loaded heavy, light, slow or fast. But you can also buy tons of factory ammo for the .40S&W.
I'm comfortable using either for any duty, target, run and gun, self defense or just turning money into smoke and noise.