Stiemsma's potential is slightly less then perks. Perk has dealt with alot of injuries already in his carrier but at his best he is one of the top 3 post defenders in the NBA. Steimsma best skill is his shot blocking but I would take an elite post defender over an elite shot blocker any day.
However i'm a big steimsma fan. I see him at his full potential be a weaker but much more mobile Greg Ostertag.
Perk overachieved ,hard to move and also penetrators were weary to drive in at times, but even at his peak, was never an elite defender in the nba imo. Only about 6'10, long arms but wasn't a very good leaper. Footwork was good but not considered fluid.
When you think of great defenders, guys like kg, olojuwon, David Robinson and Tim Duncan come to mind. And Stiesma has the ingredients similar to what these guys have .
While perk is like a chained rottweiler guardog, Stiesma is like a Doberman without a leash.
Up to you what kind of defender you want, but Stiesma imo is more versatile. Can guard more areas on the court and temporary different position players effectively