You may have that backward. The preload pressure is by it's very nature equal in both stacks. (Newton's Laws), while the forces generated by riding are mostly in the lower stack. (headtube flex acts like preload and loads both bearings equally, but steel head tubes don't flex much, if at all).

So, with loose headsets, you'd expect the bottom stack to suffer but not the top, while with greater preload you'd expect more equal wear.

There will also be differences in wear patterns. If the bearing has play, the fork will cam in the head tube, with load to the front of the lower stack and rear of the upper. and the wear pattern should reflect that. OTOH a straight head tube secured with proper preload, will sit square and the wear pattern will be more uniform around the entire race, though there may be some more wear at the front of the lower stack because of angular forces sloping head tubes cause.