Which of them do I have to pick? It is on latter box scale.
I usually pick Scale Outer. This way, if you set Window Size to, for example, 1080 x 700 but the device has screen of size 1080 x 720, the remaining 20 px will be filled with layout.
Sometimes I also add a sprite to the centre of the layout and add Scroll To behaviour. This way, the layout expands all around the screen (and the centred elements stay centred) instead of only down and right (where I have to reposition them).
Maybe experiment a bit, you will already see differences when running a preview in broser by resizing the browser.