I would be in favor of starting both Brown and Baynes and I think this is what Stevens will do.
The starting unit will already have two capable ballhandlers in Irving and Hayward. Smart's value is that he can be brought off the bench to be the primary ballhandler on a 2nd unit, and Irving and/or Hayward can play off the ball.
Baynes starts because you want some beef, defense and rebounding to start games as the Celtics did with starting Amir next to Al last season.
The closing lineup would be different. In most cases smaller with Irving, Smart, Brown, Morris, and Horford if I had to bet on it. (Maybe Ojeleye instead of Brown in some cases.) Shooting/ballhandling/passing everywhere, and be able to switch pretty much everything on defense.