Fultz.
I think Fultz has the potential to be one of the best players in the league during his career. I think Brown could be one of the best players at his position.
Fultz has elite size for his position (PG). I don't know if Fultz's athleticism is elite but it is definitely above average. His combination of size and athleticism is elite. He is young and may not be fully developed physically so both of these measurables might improve.
He has much better touch than Brown around the basket. In fact, he has the touch to be a special shooter.
His ball handling and shooting are already advanced for his age. He reminds me of a more athletic Steve Nash (though not quite the shooter Nash was). He and Nash are very smooth with terrific balance in their stop/start/change of direction rather than herky-jerky like Harden (who he is often compared to).
That said, they are both prospects. They would have to work hard and avoid injuries to tap into all that potential but if both manage to approach their ceilings Fultz will be special and Brown will be very good.