I'd say off hand that the guy at the gun shop probably had a glut of .38 specials he needed to unload, . . . or he figured you for a newbie and got a chuckle out of your inexperience.
Either way, . . . I'd find another gun shop, . . . as he clearly does not know whereof he speaks. True, . . . in a snubbie, . . . you will not develop the max velocity, . . . but you can rest assured as Ronny said, . . . the .357 will certainly whack the target a lot harder.
If you want to prove this, . . . go to a range, . . . rent a short barreled .357 and load it up .38, .357, .38, .357, etc, . . . it won't take you long at all to tell which one is which.
But again, . . . as it is often stressed, . . . caliber always has to take a rear seat to bullet placement. A well placed .22 will do more to stop a bg than 6 near misses with a S&W .500.
May God bless,
If you can breathe, . . . thank God!
If you can read, . . . thank a teacher!
If you are reading this in English, . . . thank a Veteran!