I've said it before and I'll say it again.
Brown has the potential to be a top 15 player in this league. More athletic Kawhi Leonard kind of potential. His potential ceiling is higher than Kris Dunn. Dunn has a higher floor, chances are very good that dude at least becomes a fringe all-star, Jeff Teague level, and he too has a chance of being a top 20 player, while if Jaylen bottoms out he becomes a SF version of Smart. But Brown has a better chance of becoming a superstar than Dunn, which is exactly why Ainge picked him.
This team needs a superstar, now and in the future. At #3 in this draft, Brown was the pick with the best chance to do that, even though Dunn has a higher floor.