I use a red dot on my slug gun.....it's a Bushnell Trophy. Mine is a 6 moa dot and does cover quite a bit of the target at ranges over 100 yards. Out to 100, it's not that bad.
My suggestion to you would be to look through a couple with the smaller dots, like a 3 moa, and see if they are comfortable for you. It's pretty small and would cover you out to the longer ranges. But if you're not taking shots over 100 yards, something around a 6 moa should be fine.
I've never been a big fan of halo sights....so I can't really say much about them.