The posters who are looking at it as offense vs. D have the right approach, IMO. It's anybody's guess which one will be regarded as the better defenseman, but Sekera is most likely going to have a niche as a silky smooth guy with offensive responsibility, while Yemelin's snarly attitude will see him as more of an abrasive, agitating defender.
I wouldn't read a whole lot into Yemelin's scoring at the WJC... he didn't look like an offensive dynamo while doing it, he had some seeing-eye shots from the point, etc. His rapid progression gives some room to wonder, though, but most times you hear him compared to something in the range between a Darius Kasparaitis and Vlad Konstantinov. The high-end latter of which never got 40 pts playing on a stacked team.
Meanwhile, Sekera is poetry in motion on the ice. I don't know if it's "at best" or "at worst" to say he could have an Ozolinsh-like impact in the NHL. Whether that's good or bad, it's still points in your hockey pool. And I don't say that to intentionally disparage Sekera's defensive game... I'm just saying if worst comes to worst in his defensive development, he'll still have skills and points, IMO.