I think it comes down to how you think each will improve going forward.
Simmons' ability to improve his jump shot is a shaky proposition to me. His shot was so broken in the draft process last year, that by the time summer league rolled around he basically wouldn't take jumpers. So until he confidently TAKES jumpers in real games, I'm not sold.
Fultz has basicxally every skill you want in a modern lead guard (in his insane frame). His only real question is his defense, which he has the tools to improve at. On top of that Washington implemented a zone alot last year, so while Fultz made alot of mental lapses, he wasn't put in any kind of defensive situation to succeed.
Basically I think Fultz is more likely to be passable on defense, than Simmons is to become a passable shooter. So while Simmons has a higher upside, I think Fultz is more likely to hit his ceiling, aka he has my vote. And the modern NBA requires good shooting. 10-15 years ago, I take Simmons 10/10 times.
Edit: didn't read the immediate above before posting. Basically I agree with Jpotters assessment of Simmons weaknesses, except I would rather roll with Fultz