Get both. Even better, get a couple of each.
Seriously though, if you had to choose one, I'd get an AR and a basic pump shotgun, like a Rem 870. All of the Saiga 12s I've shot have their quirks. Starting from mods needed from the get go, to how you reload, mag seating, etc. I've seen a bunch of them fail parts and malfunction repeatedly. Yeah, it's fun to shoot a Saiga 12, but I'd rather have a Benelli semi-auto.