Well, there are many good choices.
You want a hard plastic(or other material) layer to distribute pressure over a larger area (preferably to the bezels around the display)....to prevent pressure (especially at a point) on the display. This can easily happen in a bag or backpack with other objects in it.
I generally prefer the kind where you take them out of the case to use, but that is most often because good flip open cases that dont obscure ports, inputs, card slots, speakers, function buttons are often hard to find.
You can also get silicon "wraps" to use in conjuction with these slip in/out cases. Which protect the unit bezels and backsides from scratches and provide a bit of rubberized protection for drops. If you can find one that is fitted for your particular device, these are very nice.
My 2 cents.