I don't know anything about those specific models, but my Samsonite S'Cure is polypropylene and I'm very happy with the material as it's rather sturdy and does not cave much even when it's pushed when empty. Looking at the photo, Shield seems to have a rather similar structure to Samsonite S'Cure and seems to have three-point closure, which makes it pretty secure. Those cases are less prone to rain getting inside too (a hard lesson learned when QF left my suitcases outside when it was pouring it down... The zipped ones had contents wet through. The hard frame one didn't have much water inside at all, due to rubber seal between two halves.)

If you are planning to carry fragile items, those real hard cases are better (zipped ones are much more squashable).

PC (polycarbonate) tends to be more 'bendy' so to speak, and dents tend to spring back more easily, but being bendy, I find it not as suitable for carrying anything fragile.