I'd take a bit of extra weight and size for extra protection, and increased resell ability later on for upgrading.
I love how thin the iPhone is, but the idea of my iPhone dropping naked onto the pavement spooks me.
I still contend, and my experience in my own family has been, that a case-less phone treated well has a better resale value than a phone that's been in a case. The micro-scratching that phones get after being in a case for months (let alone a year or more) is the issue at hand.