Tatum is a good all round prospect with a low risk of being a bust. He can score with his shooting and getting to the rim, and has go to scoring potential. His off the ball skills are pretty good as well and plays good defense. Granted the absence of an elite attribute lowers his ceiling, but at the same time his skill set is well rounded enough to make up for that. So I don't see him being a Kevin Durant level player in his future, rather I see him being more of a Rudy Gay (with better defense) or Otto Porter (with better scoring) level player. Now that's not a bad thing because you can slot a forward like them into a line up and they will be a good option on most teams; however, they are simply a piece to the puzzle rather than the piece.
Isaac has significantly more upside than Tatum. He has excellent size and stands to be a SF or PF at the next level. His athleticism and size could allow him to guard most positions, and on the offensive end those same attributes combined with his ball skills should allow him to switch up positions and match ups to cause the opposition defense problems. The kid can shoot as well. So as a prospect he's got a very good skill set, though to be successful he is going to need to add muscle to his frame otherwise he's going to get pushed around. He strikes me as a similar prospect to Brandon Ingram from last season, which isn't a bad thing, but Isaac is another one of those players that needs to bulk up if they are going to achieve their immense upside. So there is risk to selecting Isaac and arguably he does have a lower floor than Tatum, but the flip side is that he potentially offers significantly more upside than Tatum.
Who would I pick of these two forwards? Well, I would lean towards Tatum. Like I said above, he is already a good all round prospect on offense and defense. If he continues to develop he could become a go to scorer, or at worst he could be a useful second or third option [admittedly you don't look for those at five, but that's his floor]. As for comparisons, I've seen him compared to Rudy Gay but Tatum offers better defense. If we could land a Rudy Gay like player with better defense, then we will have the small forward we desire. We already have Skal pencilled in as our starting power forward and he offers tremendous upside, likewise we have Willie and Papa who will battle it out for the starting center. So while I can understand why some would want Isaac and his tremendous upside, I would honestly prefer to see us draft Tatum because he's not only the safer pick, but arguably the better fit that allows us to keep Skal in the starting line up since I suspect Isaac will end up as a PF rather than a SF.
With that said, if the top four picks feature Fultz, Ball, Fox and one other player not named Jackson or Tatum, then do we necessarily draft the forward? Not really. I am not on the Malik Monk train because he projects to be a shooting guard and we have Buddy pencilled in at SG. So if Monk is there then I would draft a forward. However, if Dennis Smith is still available, then he would be a serious contender for our pick because he could fill our need at PG. By no means is he an elite passer, but he's a good enough passer to be make plays for his teammates. He's an explosive player with elite athleticism, so attacking the rim comes natural to him. Overall he's a pretty well rounded point guard that offers a pretty high floor.