The gap defensively gives Tatum the edge for me. Tatum is one of the elite help defenders at the wing
If you ask me, SGA > Tatum all day long on D. I would argue that SGA is already a top defensive PG in the league, along the likes of Smart, Simmons, Ntilikina, Beverley, Jrue Holiday, Derrick White (to name a few). Unlike most defensive PGs though, he's super effective on the offensive side of the ball as well.
I don't buy the argument that Tatum is an elite help defender at the wing. I mean, if Tatum is elite, what about Kawhi/George/Siakam/PJ Tucker/Draymond Green/Robert Covington etc? Not to mention, he's a below average on-ball defender. Getting steals and deflections is great, but he's straggling to stay in front of his opponents.