There is no 'visual' difference. The practical difference is the comboboxes allow you to input something other than one of the dropdown selections. You can automatically change all (or designated) dropdowns to comboboxes (or vice-versa) via a macro. For example:
Code:
Sub Demo()
Dim CCtrl As ContentControl
For Each CCtrl In ActiveDocument.ContentControls
If CCtrl.Type = wdContentControlDropdownList Then
CCtrl.Type = wdContentControlComboBox
End If
Next
End Sub