This is a good point. I cheaped out with a Maverick 88 before I had my saiga, and was dissappointed, but a nice optic for my AR and a budget shotgun would probably be a better combo than a nicer shotgun and a budget optic.
I would go with a Rem 870 or VersaMax, BUT of your 2 choices Mossberg is the way to go. Not a fan of Aimpoint.