Commuter is better than average protection, Defender is heavy duty. Also defender is the only one with a belt clip.
The Commuter is a great case. Way better than silicone, covers all outlets from dust etc. Has 2 layers a soft rubber for impact absorption and a hard outer shells that holds it together and protects from scratches etc. Comes with a screen protector as well which you can choose to use or not. If you plan on keeping the phone in your pocket this is the case for you as it doesn't add much bulk.
The Defender (imo) is the mother of all heavy duty cases. There are others out there such as the Kraken etc but durability wise i think Otter has this area covered. I have had my BB take a 3 story drop off a building in one of these and it came out fine. Not a single scratch on it. 3 layers of protection include the outer hard shell, the built in hard plastic screen protector that overlaps the screen, and the rubber absorption shield that goes over all of that. The belt clip itself is part of the defense as it encases the phone in even more plastic as it covers the entire back side of the phone. Or front/screen side as you can face the phone within the holster however you like.
Downside of the Defender is that it turns your phone into a brick because it does add a lot of bulk. Though with the RAZR being so thin it may not be that much this time around.
Both are great cases that cover all open ports from dust etc. Commuter for your average every day joe i suppose and Defender for the contruction worker types.