Aesthetically, I prefer a naked phone, but I'm concerned about it slipping out of my hand. The bumper addressed that issue on my former phone. It allows most of the phone to show but also provides a good grip.
Second choice would be a slim case. I have a thing about gorgeous iphones being placed inside a honkin' fat case that practically disguises it. Those phones just look so sad to me. But I have yet to find the slim case that appeals to me.🙁