Like I voted, Hunting + Auto.
I prefer to use the Hunting as single shot and save the Auto for big, unmanagable crowds, and bringing Scrakes and FPs to the charging point. Hunting Shottie finishes the job with a single burst, and saves ammo on the Auto shottie.
Generally, if I play right, I don't have ammo issues because I bounce between the guns even when I'm the last man standing, but I do pull out the 'nades and 9 mm on occasion to give me a bigger cushion.
All that said, there is alot going for the Shotgun/Auto combo, especially the fact that you can carry an M79 and the flashlight. Still, though, I'm always most focused on the guys most likely to kill me, I.E. the Scrakes and FPs. So, Hunting/Auto it is.