It's tempting (and mostly accurate) to think of one as an offensive player and the other as a defensive player. But it's more complicated than that. Brown is about to turn 21 and (if you ask me) showed more offensively than expected. He can absolutely turn into a top-level scorer in the next few years.
On the other hand, I think Tatum can be much better defensively than people think. Again, this kid is 19 right now. He's got length, later movement, and smarts. Don't write the book on his defense quite yet.