Both guys have some insane potential, and they both display it pretty clearly in glimpses and spurts during games. Defensively they have both bought in to the team philosophy, and they both are unselfish offensive players at this point in their career.
I think the main separating factor between them is the raw athleticism that Jaylen has, that simply cannot be taught. Not that Tatum is a slouch, and he is only 19; with some strength and conditioning coaching, the dude could possibly catch up to Brown. But Brown also has room to grow and develop.
Brown needs to work on his footwork, jumpshot (particularly off the dribble), and his handle against defenders playing tight on him. I think he has the athletic ability and has clearly made a drastic leap in controlling and harnessing his athleticism, to improve the handle and the footwork. The jumpshot confidence has made an obvious leap, so it's really just developing that rythym or "killer instinct". If he can do that, he has the potential to be about 85-90% of what made Kobe great. I actually wish he would reach out to Kobe and get him to mentor him, similar to Lebron with Greek Freak.
Tatum clearly has the more polished offensive game. Even though he has nowhere near the hangtime or leaping ability of Brown, I actually trust Tatum to finish at the rim, where Jaylen just kind of tries to rule like Dominique Wilkins, but he is still too young/timid to just own people who are athletically inferior. The lateral quickness is something I don't know if Tatum will ever catch up to Brown on, but his basketball IQ allows him to cover this "weakness".
Offensive trajectory for Tatum seems more tangible and linear, and we can visualize where he ends up. If he grows and bulks up, he will be a contributor.
I think Brown has the higher defensive trajectory, and really his determination and confidence will determine if he is the 1A to Tatums 1B in the future for this team.