You're right in that Tatum is simply better than Brown at this point in their respective careers. My (less-than-clear) point/hypothesis is that *both* are better than they would have been had they started out with dumpster-fire teams like the Knicks or Suns. And given the nature of the draft, the best draftees dont often go to good teams. (probably not especially earth-shattering, I know. But it jumps out at me with the Celtics, because they are good and they have had high picks).I think it depends on the rookie. Rookie Tatum has such a solid NBA-level skill set and feel for the game that he is probably fully capable of holding down a key role on a good team. Rookie Brown, on the other hand, wasn't playing at that same level a year ago - compared to rookie Tatum, he had more projection and less immediate value. Had Crowder, say, suffered a major injury in game 1, it would have been a lot harder for Brown to step in as an immediate contributor than it has been for Tatum to do so this year.
Its probably hard to disaggregate, but starting out on a good team would seem to maximize or speed up the improvement process.
As a basketball fan, I am somewhat concerned that a skilled player like Porzingis will have an Antoine Walker career arc because his 'formative years' will be spent on inferior teams with lousy teammates.