I use plain, clear and soft silicon cases myself. I feel the rigid the case the greater the impact forces gets translated to your phone. They'll never damage the phone and are cheap to replace. I replace the moment they start to discolour, shows signs of hardening and/or when I've dropped it and the drop has cracked the case. When the case is cracked it won't take the impact the same any more (assuming it lands in the same or similar place).
It comes down to what you're really wanting the case for.
PS The iPhone has a really hard camera bump and this bump can in fact scratch other things so something else to be mindful of in case you have say an iPad.
PPS The camera bump means that the iPhone isn't flush to flat surfaces, like desks, and as such rocks about if you use it while on a desk. If you use it on a desk a lot, like I do, get a cover that flattens this out.