A few thoughts:
As several people have said, Tatum is considered just about the best prospect in one of the stronger classes in ages. He has performed on every stage, including internationally, and been unstoppable on all of them. He has been a top player for a good long time.
Weynan is a lot more of a recently developing guy, and a guy with a different body and skill set. Very athletic, great motor, great length, but doesn't have the same basketball skills or IQ as Tatum.
At this point just about every expert on Earth rates Tatum as the better player. Obviously that doesn't mean Gabriel ISN'T good, or won't BE good, or might not even end up a better player overall in a few years. But right now, it seems Tatum is the better player, excitement aside
As for positions in college, Tatum is not expected to be a "big," although K has spoken about ignoring the traditional roles for years. Most people expect him to slot into the SF role, which should work well given his size, athleticism, shooting ability, and ability to break down a defender off the dribble. This is especially true if Duke lands Giles, as that brings in a traditional PF while allowing for a traditional C (Jeter, Obi, Vrank, and possibly Bolden.)
Gabriel has expressed interest in being a 3, especially in the League, and in college to prepare. Some players are definitely able to do that. Others with similar bodies/skill sets, however, have taken some time to adjust... think Jon Henson and Alex Poythress. While Gabriel has excellent skills for a player his size, we obviously won't know how he will play at the 3 until he does so
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He could very well succeed, and succeed wildly, or he could take a little time to adapt.