I avoid the shell type of case. I have the slide in Stilgut, which is all leather, has no shell to scratch my phone. I have used the brand on all my iPhones, and not one of them has had any scratch damage at all. The hard plastic shells are one to avoid I think.
Thing is inside is a rubber type of material but still dont understand how anyone could scratch it. I might get the defender although I know it will add much girth.
I have used the Speck Candyshell and it has not scratched my phone. I don't see how the soft rubber could unless a small rock was in there. The Otterbox Defender is a beast, but if you need full protection, this is the case.
Had the Speck Candyshell for a week, took it on and off every day and no marks on the phone. Like others said, the inside is lined with a softer material.