It won't because Harden is a top 5 NBA player and potential HOF. Jaylen Brown is unlikely to reach such heights so player for player who gives you the best chance of winning an NBA title? It's James Harden, obviously.
In terms of who makes us better on the court its 100% Harden. But then you take into account age, team chemistry and a far longer window with a Tatum/Brown duo and it becomes a tougher call.
I want Brown to stay in Boston over two years of Harden. Not least because I think Jaylen is such a great representative for this team. I want at least 5-10 years of Jaylen Brown than risking Kyrie 2.0.