Embiid would be hands down the better fit for the C's. But then again, in 5 years' time he might not even be in the league (based on his injury history).
Imo, Simmons surrounded by 4 shooters would transform into an MVP candidate. If we traded for him, we would have to reshape our roster. What this means is that we'd probably have to sacrifice Smart, possibly even Brown.
Salary-wise, Brown + minor filler for Simmons would be a valid trade this summer.
I really don't know whether it would be worth it. Gun to my head, I'd do it.
Kemba for Simmons straight up would work as well.