I watched Sweeney closely in the scrimmage, and he really couldn't move. Any post defense he gives is overriden by his inability to cover any pick and rolls, set picks without fouling, get to any loose balls, or get any lift to get his shot off in traffic.
Johnson on the other hand looked completely lost offensively, and somewhat lost offensively. But other than that actually looked decent. His shot is a thing of beauty, and he seemed to understand how to defend, even without a big frame.
I wish Sweetney was better, because he would be huge for this team (no pun intended), but he has just eaten his way out of the NBA.
Johnson on the other hand, I expect to be able to contribute by midseason, if they have a need for him because of injuries/guys not performing.