I think, as some have pointed out, it's still too early to say who will reach a higher ceiling or even, as the OP asks, who will have a better year. A lot depends on who stays healthy and how Stevens uses them.
Both have shown flashes with individual skills in individual games where, if they were to put it all together and perform those skills at the the highest level they've demonstrated on a consistent basis, would make them 'star' players. But until we see one of them both get the chance to do so and to actually _DO_ so, it's potential for either of them.
I do concur with the sentiment that, at least on the blog, Sullinger is severely underrated. I think folks overstate his weight issues and also tend to forget that he is still the 3rd youngest player on the roster (behind only Marcus Smart and James Young) when discussing his potential. And I think a lot of modern fans don't appreciate the value of strong, power low-post skills like Sully has because, well, they are rare today.
Of course, Olynyk's ability to dribble and drive is pretty darn rare for a 7 footer as well.
Both of them have high upside. I like to think Olynyk's peak upside might be a hybrid of Dirk & McHale. I'd like to think Sullinger's might be Wes Unseld with an outside shot.
But upside is not always reached.
I do think both have established that their 'floor' is at least as rotation players, and that's pretty solid to build on.