If we don't end up with the first pick (Fultz), I'd be more inclined to take Jackson because of his higher potential. I do see a more similar development to Smart or Winslow, but possible a year earlier. He will find minutes because his hustle and defense and will contribute to winning through the invisible intangibles. Tatum, I see as more of a scorer that will be more in the mold of Pierce or Anthony, but not until his 2nd year will he break out. He has the scoring arsenal of a 1-on-1 isolation specialist, using the triple threat, primarily. His success is less certain as he would struggle to find court time in Stevens' system because of his defense, even less than Brown.
I also think Jackson would fit better next to Smart and most importantly Brown. Brown can slide to the 2, while Jackson's size precludes him to favor the 3. The 4 and 5 are positions that will be addressed likely via trade.