Well I have an Extrema II and love it. Mine cycles everything I have tried in it. Even 1 oz loads in 2 3/4". Do you need to shoot 3 1/2" shells? Nope. But my gun patterns very well with them so that is what I shoot. I can so I will.
As for the Mossberg I have no experience. Sorry. FWIW I haven't heard a bunch of bad stuff. That can't be too bad then.
I would say that when you buy whatever shottie you decide on, take a bunch of different types of ammo and try them. You will be surprised at the differences in patterns.