Lots of choices out there. I have a Bushnell red dot on an M1 Carbine that is very compact and only about $150 so it is cheap enough to try out and see if you like it or not. It seems perfectly serviceable. I would agree on the idea of also getting backup iron sights suchas the Magpul or other brand.
On my home defense AR I have an Aimpoint PRO red dot with a Magpul flip up rear sight. The PRO is the best value in very robust, full featured red dot at $400, I think. It is 2-3 times bigger than the Bushnell but much shorter than even a compact scope. Plus, you don't need to have perfect cheekweld position as the red dot is on target regardless of the angle you view it.
A red dot is great for out to 100 yds for very fast target acquisition, moving targets. The closer you expect to shoot pigs the more I would recommend a red dot. But a low power optical scope can be pretty effective as well especially if range is 50-200+ yds. I have a Weaver 1-3x scope on another AR and like it a lot but for under 100 yds I would favor the red dot.
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