Not in any way being a shotgun expert, but liking SxSs (probably for the nostalgia and seeming simplicity), there seem to be a couple of problems with them that I have a hard time ignoring aside from, but not unrelated to, capacity issues. Even if I get "pretty good" at reloading quickly and under stress, it still requires many more intricate movements requiring dexterity, i.e. fumble-prone movements, than operating the slide of a pump shotgun. The second problem (for me) is the state of readiness. The SxS must either be kept empty with shells nearby, or loaded but broken open, or loaded with (internal) hammers uncocked (possible on some guns but meaning the gun must be broken open to cock the mechanism before use) or loaded with safety on (not safe in my estimation). Plus, unless modified, each time the gun is fired and reloaded the operator must train to take the safety off again (another step under stress). Am I the only person here who has ever lost a bird because in the excitement of the flush I neglected to take off the safety? (I bet I'm not. I know it's training but it's still something that can happen.) It just seems that a pump gun with reasonable capacity, loaded magazine, hammer down on empty chamber, safety off is easier to run with less chances for screwing up. I like SxSs but my brain tells me an 870 does a better job.
Still, my SxS is a Remington Spartan 220 (Baikal), double trigger, cylinder bore, 12 gauge. I like this gun because I can release the (rebounding) hammers and still keep the chambers loaded but have to crack the action to cock it for use. Still faster than loading the gun.
Plus, the SxS is secondary. I have an 870P as my main defense tool.