I think for me, it depends on what I am fighting, where I am fighting, and what's available. For example, if I am playing close to the end of the "Pillar of Autumn" level in Campaign, I'll opt for an Assault Rifle and Shotgun, due to the rapid fire of the rifle to mop up Grunts and to down Elite shields and then switch to the shottie for the stopping power against the Field Marshall.
On topic though: I'll have them both. Although, I suck worse than Church with the sniper rifle. Compared to him, he can actually hit a target.