I do expect Smart to be starting this year at the SG spot.
While an argument can be made for both players, I think that Smart's knowledge of Brad's system (as the longest tenured Celtic) makes him a very important piece in that starting five.
Throw in his ability as a defensive stopper on the perimeter (which I feel this starting lineup really needs) and his general hustle, and I think he clearly makes the most sense at that position.
Especially with Kyrie, Hayward and Horford on the court, that gives us at least three guys out there who can all score, shoot, handle the ball and make plays - the perfect lineup for hiding Smart's offensive weaknesses.
Meanwhile if we start Brown then out bench would consist of Rozier, Smart, Tatum, Theis and Baynes - that's nowhere near enough offensive production off the bench, with Tatum being pretty much the only guy in that unit who can score consistently (rozier when he is on) . Starting Smart allows Brown to go to the second unit, which is much more balanced with Rozier, Brown, Tatum, Theis and Baynes.