I have both and I agree that the defender is not that much bigger than the commuter. The commuter to me seems less refined than the defender overall. My commuter, that I'm returning, had some issues with molding of the rubber. My defender was perfect.
I agree with you 100%. My Commuter's fitment was a joke. I have seen several pictures of the Defender in actual use and the rubber fits perfectly around and in the plastic shell. It looks a lot better than the 4S version.