It depends what you want, if you take player potential into account, and if you take team potential into account.
Right now, Sully is the marginally better choice. He is sligtly less effective at the offensive end (I still don't trust his 3s), but and his strength and bulk are excellent defensive attributes. I'd give him a 6.5 Offense, and a 7.5 Defence.
I feel like Sully's cap is an Al Jefferson type. A strong traditional Power Forward who will get you rebounds and points near the hoop. If you want a traditional power forward on your team, Sully is your man.
HOWEVER
Olynyk is not *that* far behind Sully defensively right now. He put on muscle over the summer, and looks much more comfortable than he did last year when it comes to guarding stronger post players. And as much great as Sully is offensively, Olynyk has more crafty tools on that end. Olynyk's passing, shooting, driving, and creativity on the offensive end have him at a 7.5 Offence. Currently, I'd give him a 6.5 on defense.
Olynyk's potential Cap as a player though is a Dirk Nowitzki type player. Olynyk has the potential to be an utter matchup nightmare. Maybe I've drunk the punch on this one, but I think that Olynyk has all-star potential, and would be far more valuable in a modern NBA style team moving away from low post bangers to three point shooters/divers. Olynyk also has the height to play the 5--Something that Sully will never have. Olynyk changed his body quite a bit over the off season, where Sully still looks like his same effectively pudgy self.
Do you want a player who is 60% of the way to being Al Jefferson, or a player who is 50% of the way to being Dirk Nowitzki? Remember. There is no guarantee that either player will progress any further than they are right now. The question on who is better depends entirely on the direction you want the team to take, and how much faith you have in the player to realize their potential. There really isn't an objectively true answer to this, but I, ever the optimist, am going with Olynyk.