My bet is still on KO reaching his potential, which I think is higher than JS will get to.
You can see the many areas where KO is still learning and, given his reported work ethic, will likely continue to improve upon.
But that's certainly not to trash Sully as a player, as he's got some strengths too. Sully's been improving a bit too--e.g., in 3PT% and assist rate--but he's down on rebounds and more or less lateral overall for his 3 years in the league. (Efficiency is still down from his rookie year, when he had a significantly lower usage rate.)
In short, Sully hasn't been developing better or even as much as KO, but he came into the league as a more composed player, prepared for his initial role on the team. I don't see Sully changing significantly as a player unless he improves his body, so as to become less unathletic, if you will.
One big difference that is seldom noted on Sully's game is his lack of offensive efficiency. His TS% this year is .516 to KO's .583. From 0-3', KO's shooting .746 to JS's .662. And only KO is also a legit 3PT threat.
That said, I think this is a big summer for KO. Either he goes to a really tough big-man camp and becomes less of a finesse player, or he essentially tops out as another Bargnani.