Poor man's James Harden offensively with much better defense. I say this because of their similar shot selection on offense. It's pretty uncommon to see two players rely on 3PT shooting and driving to the rim with barely anything in-between. Neither player relies on explosive athleticism to drive to the rim. They're either crafty or bull their way through. The difference is James Harden was a much better 3PT shooter in college (5% better), and this is why I'm primarily saying he'll be a poor man's version. Remains to be seen how Smart adjusts to NBA length and whether his 3PT shot gets better or not, but stylistically I feel comfortable saying Harden.
Smart took 42% of his FGA from the 3PT line in college, while Harden has been hovering around 40% 3PAr (not 3PT%) in the NBA. Smart had a 65% FTr (not FT%) in college while Harden had a 60% FTr in college. Smart took around 45% of his FGA at the rim (meaning ~87% of his FGA were either at the rim or the 3PT line; barely anything in-between) while Harden takes about 30-35% of his FGA at the rim in the NBA (not sure about college) totaling to about 70-75% of his FGA coming from either the rim or the 3PT line.
Both are above average playmakers that can run your offense (though I don't like what Harden has become in Houston honestly). OKC relied on Harden to be their point guard down the stretch because of how erratic Westbrook can be.
I think Baron Davis is a good comparison overall (big point guard, lots of 3s, shaky shot selection, great defense, etc), but he didn't get to the FT line as much as Smart projects to. Smart's foul drawing ability is one of his biggest pluses. That and his 3PT chucking is what leans me to more of a Harden comparison (for offense). Most 3PT chuckers (and I mean this in the best of ways) do not drive to the rim and draw fouls as much as Harden and Smart do.