I think Fultz currently has the more complete game but Simmons offers a very rare skillset for a forward. he reminds me a lot of Magic when he's got the ball and driving up the court. shoots poorly like Magic did when he first came to the league but he worked it out after several year.
Fultz could be a multi-time All-Star but if Simmons puts it together, I think he could be a top 10 player in the league.
I think if either put it together they have Hall of Fame potential. Simmons has a unique skillset, which if he adds even reliable (but not elite) shooting to will make him an incredibly difficult player to contain. Fultz, meanwhile, has the scoring ability to allow him to be a perennial top-scorer in the league once he gets his pro legs under him. Playing even average defense similarly moves him to that true franchise status.
Again, this is about the potential for both, not what will happen. Adjusting to the pros can always be difficult, and they both share a trait in that their competitiveness is currently somewhat subject to question based on poor results from their college teams.
I would note the Fultz is practically two years younger than Simmons, which gives him more time to grow both physically and basketball-wise.