From my experience the Iphone is the easiest to take pictures. For someone like me, who isn't experienced with cameras; it makes it easy to get good photos. I think this is why the Iphone is so popular with it's camera. It's easy for the average person to just point & shoot. I have a Note 5 and, if I'm careful I can get some really good pics, even better than the Iphone at times, but it still sometimes requires more effort.
It's not a big deal for me, but i think that's why a lot of people prefer the Iphone camera.
As far as smoothness. The Iphone used to be the best. It was buttery smooth throughout, and one of the things I loved about IOS. The Iphone 6 Plus is still smooth, but not like I remember IOS being in the past. The music app is choppy when rotating and scrolls are choppy also, it's just not the same.
Android has come a long way. Right now my Nexus 6 on 5.1.1 is running better than it has ever ran, and is the best running Android phone I own as far as being lag free and smoothness. My nexus 9 just updated to Android 6.0 and to be honest, it was smoother on 5.1.1 I'm hoping my N6 still run good on 6.0, I know it did when I tried the dev preview.