The carbine beats the pistol hands down in being the most powerful weapon that has plentiful ammo, sure the automag has style, but the carbine doesn't need to rely on the second weapon to take down shields and armor. And of course it gets head shots just fine. Also the automag doesn't have very plentiful ammo unless you have access to nearby supply caches.

Since the carbine keeps me covered in any situation, I can feel free to vary my second weapon as much as I want and change things up to keep things fresh without having to worry about "Will this weapon combo get me through any situation I might have?" because the carbine and a couple nades can get me through anything.