I'm torn myself with the same question being the deciding answer.All depends.
If we are still a civilized nation per say I would say a pistol. If I am in a situation like the movie I am legend, I'd pick an AR or AK.
I would probably take a 9mm pistol like a Glock platform that can be converted into a PDW so I can have more versatility.If you could only own one gun the rest of your life what would it be and why?
What Brand, model, & caliber.