I've done a crazy thing and took a few test shoots with my 6S against my professional Nikon DSLR (Nikon D800 and 28mm f/1.8G prime lens)
My take is that the 6S is surprisingly good at 2k resolution. So if all you need is electronic viewing/sharing, there is a very little actual difference. Obviously, the D800 is much better given by resolution and dynamic range of the sensor, flexibility, you can put more lenses on it, better controls, etc. I wouldn't even dare to do any testing in low-light as there would be no competition. On the other hand iPhone is easier to use and I love its workflow.
test is here:
goo.gl/F6oTM1
Enjoy
My take is that the 6S is surprisingly good at 2k resolution. So if all you need is electronic viewing/sharing, there is a very little actual difference. Obviously, the D800 is much better given by resolution and dynamic range of the sensor, flexibility, you can put more lenses on it, better controls, etc. I wouldn't even dare to do any testing in low-light as there would be no competition. On the other hand iPhone is easier to use and I love its workflow.
test is here:
goo.gl/F6oTM1
Enjoy