In my experience a hard plastic case will not scratch your device any more than a soft TPU/silicon type case. If properly fitted, the case will wear the iPad finish at the contact points and this will be same with soft or hard case. The primary concern with ANY case is the debris which will get trapped between case and device and will touch/rub/abrade the finish. This will happen regardless of the rigidity of the case, but some say a flexible case will cause MORE rubbing. The key is to clean between case and device frequently.
Hard cases are less preferable primarily for shock resistance and their tendency to pass the impact along to the device. Softer materials absorb forces and only the remaining force is passed along to the device.
I have the ESR smart cover compatible hard shell case and like it. Currently using it. I rarely drop my device though and am only looking for protection from tables, concrete, bricks, dirt, etc. where I might set it down. It has a nice matte finish on the back which hides fingerprints.
I also have the Otterbox Symmetry case for the Mini ($60). It has soft silicon or TPU type borders with a more rigid back, but not as hard as the molded hard plastic of the ESR. But the Otterbox is thicker, and I don't care for the placement of the pencil elastic holder.
I want my iPad to be in the Otterbox when I drop it, but in the ESR along with smart cover when I use it.......
