Watching the videos of both, it's a case of offense vs defense. Tatum is very smooth offensively, and has the length to compete at the 4 if he gets stronger. The kid looks like a natural shooter in the post/outside, and is probably going to grow to 6-9. He can block shots with his reach. Jackson looks more like a Crowder type. Don't see him as a big scorer, but mainly a scrappy defensive player.
If the Celtics keep the pick, Tatum would be the fit. If they trade for a Butler or George, the pick will go with the deal.
I also looked at Isaacs--a wiry 6-11 who looks like a project but may be the best rebounder and shot blocker.