I like both, but I'm definitely on team Tatum - and (I feel slack for saying this), but it's not really very close.
I look at Brown and I see a player who has shown flashes of competence in a wide range of areas - defence, passing, shooting, slashing, rebounding, etc. But to be completely honest, he hasn't really shown me EXCELLENCE in any one area - and I feel you really need to be elite at something to be a star player in this league. I don't feel that being "pretty good" at lots of things really gets you there.
Tatum has some potential weak points in his game, but none that are significant enough to really concern me. Yet his ability to score the ball is truly special. His level of footwork and the array of moves he has...he might well be the 3rd most skilled offensive player on this Celtics roster, and he's a 19 year old rookie! His offensive upside is through the roof - offensively he pretty much has no floor. How special he becomes I think will be determined by how well he can hold up in other areas (defence, rebounding, playmaking).