I actually rather have Garland over Brown regardless. Here's things Garland does. He can dribble the basketball. Brown literally can't. He can create his shot, get to the basket, and is one of the best pick and roll guards in the league.
He doesn't really have a flaw offensively. His defense is not that bad, either. He's not good, but I wouldn't say he's dreadful.
Am I saying Garland is flawless player? Absolutely not, he needs to cut down his turnover rate, and play with a lot more discipline.
I rather Garland tune his game up. He's still young, not even in his prime. He's still just 23. Why are some that down on him? He can't get better?
If I were Boston, I'd do it. I wouldn't if I were us. I think we'd be worse short-term, and long-term.