This is extremely tough for me. While Smart is the heart and soul of this team, Brown just gives the lineup so much versatility. Contrary to the sky is falling threads regarding Brown, he has looked much more like the Jaylen Brown of old for a little under half a season. He still has fantastic positional size, and has already shown a commitment to playing defense at 22 years old.
If we are assuming Hayward rehabs and eventually makes it to the hypothetical starting lineup, he gives us a true secondary playmaker. In a lineup of Irving/ x /Hayward/Horford/AD, where x is Brown or Smart, I like Brown better. That lineup has enough "plus" passers in Irving/Gordon and Horford that the ball will continue to zip. They could better use a knockdown 3pt shooter with good size than a hustling passer. Brown fits that bill better than Smart. He is having a bad shooting year, but he is already having positive regression towards his career averages (38%.)
This isn't to say it would be easy to let Smart go. He brings a ton of positive things to our team.