Even with SHTF you need to plan for the most likely scenario. Are you gonna be running around the wastelands picking people off at hundreds of yards or are you gonna be held up in your house defending at your doorstep? That's a whole other thread but you get the idea. If shooting minute of man accuracy at 200 yards is good enough the go the red dot route. If you want to punch smaller holes go with a 1-6 or even 3-9. 3-gun setups aren't necessarily practical for home defense. They're built that way for a specific purpose where the primary goal is speed and target acquisition at a certain distance.

What I would do...get the red dot as that will cover most of your bases (unless you want to be a super duper sniper) and then build another upper that is more so for precision. Then, when the zombies attack, you have your bases covered.