Tatum has more natural talent and a higher ceiling, but Brown seems to have a higher motor, and imo that counts for a lot. He's been given a new role every year and found a way to adjust to it (it took a while this season, but he got there), whereas we still have serious questions about Tatum's willingness to adjust his game to fit the other pieces on the team.
If Kyrie stays and Ainge commits to building the team around him, we should trade Tatum. Those two are both iso scorers who won't mesh well together unless Tatum develops an effective off-ball motion/catch-and-shoot game the way Jaylen has, and I'm not sure he's willing to do that, given the mentality we've seen from him so far. Brown is a much better complementary scorer alongside Kyrie.
If Kyrie walks, the question of which Jay to keep becomes more complicated. But in the end, it's about who New Orleans wants more.