I just checked your sample.
In simple words I would say:
InLine automatically chooses the correct editor (even in a bit more complex cases)
PopUp needs "manual support" since it is only able to choose the correct editor for very simple things (like ForeignKey-DropDown)
Or to me more specific - with InLine mode I get "RAD" (RAPID...) with PopUp I have to fiddle with templates and such things.
Are there any plans to enhance PopUp mode so that it is able (out of the box) to choose correct editor just by using "Datatype" attributes (or UIEditor hints) like Inline mode does?