I like them both but Tatum is a ball stopper and while both are often playing out of position at the 4, Jackson IMO is the better defender. jackson can guard 3's and 2's, Tatum can't, so unless you're drafting him as a 4, you have to find out if he can defend at the NBA level as that's likely where he plays unless he grows a bit more.
I like both, I'm a big Jackson fan he's just one of those kids that does all the little things that help you win. Tatum to me is a bit more selfish kid although he seems to be playing better of late so maybe it was getting used to his teammates.