Giannis, Age 19:
77 games, 24.6 MPG, 6.8 points, 4.4 boards, 1.9 assists, .44 from 2, .34 from 3, .463 EFG, .68 from line
Tatum Age 19:
80 games, 30.5 MPG, 13.9 points, 5.0 boards, 1.6 assists, .475 from 2, .434 from 3, .538 EFG, .82 from the line
Tatum is a far better and far more efficient player than Giannis at the same age. In fact, while Giannis took a major leap to .51% from 2 his second year, it has taken him four years to get to Tatum's Rookie efg of .538, he's still behind Tatum in FT% and three point %, by a wide margin.
It is hard to tell what Tatum's ceiling may be but with another 10-15 pounds of muscle and another year of experience he may well be in the Zone of where Giannis is today, given the minutes and shots.
Lastly, and perhaps most importantly, from what I've seen it appears that Jayson may be more of an alpha dog mentally. He looks like he has that killer instinct, along with elite shooting, something Giannis sorely lacks.