I made this declaration on draft day last year - Terry Rozier has more upside then Marcus Smart. Not just a little bit more upside either - a LOT more upside.
Rozier is around 6'3" (not the 6'1" people often list him at). He has a 1" shorter wingspan than Rajon Rondo. He's got good physical strength.
He measured a 38" vertical at the combine and some of the best athleticism measurements (lane agility, sprint, etc) in the draft that year. Very good at changing diections and changing speeds with the ball. Can stop on a dime, then explode to the basket. A standout athlete, in the mold of Leandro Barbosa.
He is a good ball handler - not an elite one, but a good one. His ball handling combined with his explosive athleticism allows him to get to the rim extremely easily.
He has a an underrated midrange game - he's very good at driving hard towards the basket, stopping on a dime, and then pulling up for the midrange jumper. That was one of the main weapons in his arsenal in college.
He's an underrated outside shooter as well. He only shot 30% from three in his last college year, which led to criticisms about his lack of a jumper. However he shot 37% from three in his freshman year, he shot 41% overall from three in the 15-16 Summer League, he shot 34% from three in the D-League. and he shot 36% from three in 5 playoff games last season. He isn't a standout shooter from three and I wouldn't even say he is a good one...but he is a decent one who seems to have the potential to become a good one.
He was known for being an outstanding defensive player in college - he uses his athleticism, strength and length very well. Add in his very high motor, strong work ethic and tough attitude and he has pretty much all the bases covered to be a good NBA defender.
He's a standout rebounder for a guard. Very impressive instincts going after boards, and is almost Rondo-esque in that regard.
He's not a natural facilitator and is not a standout passer or playmaker, but he is good enough to get the job done, in a league where pure playmakers are honestly pretty rare.
It's Rozier's physical attributes combined with his versatile skill set that gives him such a high upside in my eyes. When you have a kid who has elite length and athleticism...who can hit the three, score from midrange and get to the basket at will...who can handle the ball, defend, and rebound at a high rate...and who has outstanding intangibles...it's hard not to see a lot of potential in a player like that.
Rozier doesn't need to reinvent the wheel to become a great player - he has all the foundations in place, and really all he has to do is steadily improve a bit at a time in each area, and in 2-3 years he'll be a good starter or perhaps even a borderline star.
With Smart it's harder because he doesn't have the type of versatile all round skillset Rozier has. His defense is standout, no question about that. He's a very good rebounder for a guard. His playmaking isn't a standout, honestly is probably about on the same level as Rozier.
But Smart's lack of scoring ability is a limitation. He struggles to score at the basket, he doesn't have much (if any) of a midrange game, and we all know that he struggles immensely from three. He's done a better job at getting to the line (and shooting better there) which is something...but he's still very limited as a scorer, which ultimately holds back his potential.
His lack of elite athleticism and his so-so ball handling ability also limit his potential somewhat, because with neither of the above it becomes difficult for him to get shots off on a consistent basis.
I don't think Smart has the core foundations in place to ever become an above average scorer...and when you are a PG/SG who isn't a standout passer or a competent scorer, that makes it hard to become much more then a good role player tbh.