I have shot both and would pick the Beretta for myself. If you are looking at the SBE and a 391 in the same ballpark for price I am guessing that it is the Extrema. Take a few minutes to go over to waterfowler.com - disussion forums - waterfowling firearms. Do a little reading on the SBE and the Extrema both, neither are having a stellar record so far. Also bear in mind that if it can get screwed up on a gun a duckhunter probably figured out how to screw it up. The 3" 391, and the 3.5" SuperX2 are the autos that have the least complaints, at least they are not ripped on a daily basis. The Extrema complaints I have seen so far have mostly been poor shoking and/or barrel boring, lots of them have been shooting a foot to a couple feet left. It has been pretty consistant info from across the country so I think they do have a problem. If it really tickles you buy it with a guarantee that the POI is decent and the extraction is 100%.
I had a Browning Gold 3.5", loved it. Shot it pretty well and it ran like a swiss watch. I could not bring myself to knock the glare off it, so I sold it. Bought a SuperX2 for my "do everything shotgun" and it was THE BEST shotgun purchase of my life!!! It is one heck of a shotgun, reliable and tough. It also fits me well which makes it very effective for me, we all love a shotgun we can shoot well. I have yet to even handle let alone shoot a shotgun that comes close to the SuperX2 for me.