I'm not into firearms (though not opposed to them), so I don't own any other Magpul "gear." However, I have had a Magpul Field Case on my iPhone 5s for the past two years, and the phone has been dropped from waist-high onto asphalt, concrete, ceramic tile, and hardwood with no visible damage to the phone OR the case.
The Bump Case combines the flexible TPU material of the field case with a less-flexible polycarbonate outer shell. It appears to be the same size and shape as the Field Case. I bought the Bump Case a while back but have never needed to use it.
JKG