I have Magpul MBUS on my AR but none on a shotgun. They lift your line of sight quite high off the barrel, something I personally don't like in a shotgun. Hopefully others have them and can give a better insight.
I had some on my Bullpup, they worked well, however I had a full length rail on it, they will raise your POA so you will need to change your cheek weld somewhat.
I like the open fiber optic Williams Fire Sights. They are small but very visible and they have a low profile. My gun has a rail but they make a variety of similar models for shotguns. They are a nice compromise between quick target acquisition and 'aiming'.