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I've never owned an Otterbox, but a friend of mine had the Impact, & it ripped in several places after only a few weeks. Not sure if that's typical for it.
As for me, I'm not a silicone case fan AT ALL, so I will probably go with the Defender or Commuter. I'm pretty much always sliding my phone in/out of my pocket, & silicone makes that hard to do.
Another case that offered me GREAT drop protection with my 3G & 3GS was the Switcheasy CapsuleRebel. They haven't released it for the 4 yet, but I really suggest it. I dropped my old phones on concrete, hardwood, & tile SO many times, but didn't even get so much as a scratch on either phone.