You can load them both and tailor your loads to the cases and intended uses. I have both .38 and .357 loads concocted for paper punching, for steel plates, for hunting, for my rifle and for defense.
All it takes are a few twists of the dies and a change in the powder charge and you can go from .38 Spl. to .357 Mag. and vice-versa, so there really isn’t much extra effort involved.
But, if you want to shoot only one cartridge length you certainly could do that, too. Just be sure to mark them with a sharpie, or use brass for target and nickel for full power (etc.), so you can tell the different loads apart.
Stay safe.