Both have their weaknesses. Jackson will be better than Marcus Smart, he's an infinitely better athlete, but it shouldn't surprise anyone if JJ didn't turn into a good offensive player, and frankly, that's what matters most. We need shooters and a guy who can get his own shot when he needs to, and Jackson doesn't have that yet.
Neither does Tatum, but he's closer to it. He has a good all around game, but the concerns about his lack of quickness and him being good at everything but great at nothing are well-founded. This is why passing on Fultz was such a gigantic mistake. We need an offensive playmaker other than Isaiah, and we gave away the guy most likely to become one, in favor of 2 guys with big question marks in their game.
IMO, past Fultz, the best player in this draft is Dennis Smith. I don't think there's any way ainge takes him without trading down, but apart from his problematic attitude, he has a game that if refined, can be without weakness, and he's a beast one on one or in the open floor.