Hi Marlin,
There are two different preview modes.
One is the preview mode for the object itself. This can be set in the object node, or by context clicking on the object in the 3D Preview and choosing an option from the Object Preview Mode submenu. It defaults to wireframe.
The other preview mode is for the 3D Preview itself. This can be set using the preview mode button at the top of the 3D Preview. This button has a little blue cube icon, which is the icon for objects. The default setting is bounding box. The preview mode of the 3D Preview overrides the individual object preview modes.
The reason there are two settings is basically for performance reasons. Wireframe mode can be slow, especially close in to objects and populations. For example you might want to switch the 3D Preview mode to wireframe to position objects but then change it to bounding box (or even to hide objects) to navigate around your scene. Being able to override the individual object preview modes with the 3D Preview means you don't need to go to each object's node and change the settings when you want to do something like that.
Regards,
Jo