It is a difficult choice, because both players are so unique. Both are the same age even though Marcus has more NBA experience.
Triple double aside, Rozier is a 6'1" not-really-point guard (a dying breed in today's NBA), erratic shooter, but he brings a level of athleticism that surpasses 99% of players in that genre. And more than just athleticism - I think there's plenty of skill involved in hauling down the number of rebounds he gets as a 6'1". If he leaves Boston, his natural fit would be with a team with a taller PG or one that doesn't depend on a PG to run the offense. Philly with Simmons, wherever Lebron goes, the Rockets with Harden, etc.
Smart is harder to peg, because he his set of skills and weaknesses is even weirder. His PG skills have progressed since his rookie year, while most of the rest of his offensive game hasn't. On defense, he brings a ton more value, including the ability to match up with a wider range of players than one would expect, although I don't necessarily think he's a great 1-on-1 defender against some of the elite PG types (ie, Kyrie).
I could see Rozier being more of a trade asset this offseason because of the extra year before he hits FA.
I think Ainge's approach here should be to let the market dictate what he does. This offseason, Smart hits RFA and Rozier has one more year before RFA. Smart either gets a long-term deal, from Boston or elsewhere, or takes a 1-year RFA deal that is his bridge to UFA. Ainge's move here, I think, is to let him test the market, and to have a specific dollar figure in mind that he won't exceed on a long-term deal. If Smart doesn't hit that target in RFA, then I think Ainge can be open to either approach: long-term deal with Smart at his preferred price... or 1-year bridge deal.
The approach to Rozier will be similar (have a dollar figure in mind and then go through the same process as with Smart) but may depend on how the Smart situation shakes out (and the Kyrie situation also). If Kyrie and Smart re-sign, the openness to moving Rozier or letting him go will be greater, obviously.