One of the benefits of using the volume rocker is that you do not have to be looking at the screen to take a picture. As there is not haptic (vibrational) feedback when you press the on screen button, you don't always know if you took a picture. The rear facing camera is way better than the front facing camera, so a selfie would look much better with the rear facing camera.
As a general rule, I try to avoid using the rocker because I can touch the screen much lighter than I can press down the volume button. A DSLR lets you get a full grip on the camera to stabilize it, but it can be difficult to use both hands to steady a phone camera so that it doesn't shake when you press the rocker.