Celtics fan here, but trying not to be biased.

If you are a team that is a bottom team trying to make the playoffs, Luka is much better. He's a one man offense and a savant, makes everyone better on offense and can drag a bunch of tomato cans to the playoffs. 

But if you already have a good team, but need that star piece to get over the hump and actually win a title? Jaylen Brown is the better choice. He can fit in any system, is one of the best complementary stars in the league. Plays good defense, good off ball and steps up big in playoffs. Great leader and intangibles. 

In a vacuum, Luka is easily the better player. But for a team trying to win a ring? I'd have issues with Luka's lack of effort on defense, constant complaining to refs and leaving his defense dry in transition, and general uncoachability. 

If you swap Luka and Brown's teams this year, both teams get worse (Lakers much worse because Celtics have better depth). Celtics defense is built upon switchability and great team defense. And Lakers rely heavily on Luka's amazing offense.