It doesn't matter as long as you can hit where you are aiming. The biggest consideration is fitting the caliber and bullet type to the occasion. Such as in the house self defense or outdoor carry in the winter time.
Just be confident you can handle what you carry and know your legal rights.
For defensive purposes, 3" and under Barrel in 38 Special I like the Gold Dot 135 Grain +P "Short Barrel Load".
In .357 Magnum I shoot only 125 Grain JHP's or SJHP's. Even out of a shorter Barrel it packs more "wallop" than a 38 Special out of the same gun.
For the 4" and longer Barrels in 38 Special, I have no clue, as I don't shoot 38 Special except in my "Snubbie" 38 Specials.
The main thing is being able to hit your target and have the ability to get fast follow-up shots.
Think about what you are trying to accomplish when you train or target practice. The main issues should be: handling the weapon enough to be totally familiar with all the "controls" plus drawing, loading, and unloading. When shooting, you want to refine your sight picture, establish trigger control, grip, and stance. None of these are "ammo specific". I would recommend buying affordable ammunition with light to modest recoil. Shoot a decent amount then run a cylinder or two of "duty loads" through the gun just for grins.