As a gun owner, police officer, SWAT, and ex-military, I can tell you that the 870 is clearly superior to any shotgun already mentioned, including the Benelli. I have 2 Benellis, one being the 14 inch model. I also own the Beretta 1201, an 870, and had a "Mossi."
In a gunfight, the professionals I have worked with have always preferred the 870 for it's simplicity and well-known reliability. I've been in dozens of SWAT classes and military bases, and I can say that I've seen about a dozen Benelli's break or hangup during the course. I can only remember an 870 hanging up once, and I think the operator simply short stroked it- although they adimmantly denied it at the time.
Regarding accessories, the speedstock is a definite NO. The shells tend to fall out because of recoil or manipulation. Side saddles, ghost-ring sights, and a Sure-Fire light mount would complete an already superior weapon. Stay away from pistol grips and folding stocks. Learning to handle weapons in a "universal" manner is worth gold when it really matters.
Just my .02 cents worth from past experience and several actual encounters with the "elephant."
***ptp