Rubber/silicone cases seem to offer better protection. By default, it'll soften the blow of a dropped iPhone. However, I absolutely HATE the fact that when I take the phone out of my pocket, everything else comes out too. Therefore, I experimented with the hard/slippery Silicon iPhone 4 Cases (such as the incase slider), but sometimes that doesn't seem safe.
   Recently, I found the Otterbox Commuter Case TL, which offers both! The skin is rubber and the edges are padded. On top of that, there's a hard shell (which also protects the Apple logo, by the way). It's the best of both worlds. I also like the fact that it covers the ports.