Josh Jackson is shorter, has shorter arms, is older, has a worse jump shot, and is not as polished when it comes to creating his own shot in one on one situations.
Tatum has a much higher ceiling.
Josh is a better defender, rebounder, athlete and playmaker. Basically the only thing Tatum is better than him at is overall scoring.
Right now Tatum's game commands him to be the #1 option, and you will never win anything unless he's your #2 or #3 guy.