Don't know about rarity, but round barrel is pre-war and ribbed barrel is post war.
That said, round barrel with a half moon front sight (and fixed rear sight) is a "Heavy Duty" 38/44 rather than an Outdoorsman (which is a target grade gun).
Ralph Tremaine
And SCSW notes more post-war Outdoorsman produced than pre-war, and the fixed sight Heavy Duty model with more than either Outdoorsman. None of these would be thought of as rare---not even scarce. Some early post war examples fit with pre-war magnum barrels (ribbed) are thought of as rare "transitionals". The more practical realists think of them as examples of S&W's penchant for using up obsolete parts before moving on.