Marincin for me.
I think both are very similar in talent.
Marincin is very clearly the superior player right now and is playing like a true top 4 NHL D-man right now. At only 22 right now he is years away from his peak, plus has so much room to grow physically. Imagining him being 210-220 while maintaining that great skating, positioning, and reach... The guys ceiling is crazy good.
Really depends how much he can improve his strength/weight. But really with proper diet and work out that should not be a problem. Very much like RNH his main draw back is not being big enough to handle the big western conference.
Klefbom's effectiveness this year has been greatly exaggerated. A lot of fanboys are pumping tires. You can not even really compare the two since Klefbom primarily has faced much weaker opponents, lower ice time, and done slightly worse in a much more limited role.
Klefbom does have the draft pedigree, potential, and physical tools to be better than Maracin but right now that is purely speculation and hope. Disappointing would be the only way to describe Klefboms career to date. Injuries and adapting to the NA style of game has slowed his development to a crawl. Barring more injuries he should be able to get back on track but I fear what the loss of a full year has done to his top end potential.
I think Klefbom is one more serious injury away from becoming a bust... Btw last I heard he was injured again down in OKC.