Big fan of 2-7x variables on slug guns, or even a 4x. I would second the Prostaff.
I have used red dots on smoothbore slug guns and I would say my personal max for a non magnified red dot would be 50 yards or so. They are bad in low light as well (you can see the deer until you look through the red dot, then you look through the red dot and see nothing but the dot and a black backgroud).