The M3 was conceived for law enforcement, the original idea being that semiauto would be used for normal operations and that the pump action would be used for specialty rounds that won't cycle the semiauto, such as beanbag rounds.
Turns out though, that when people developed repeating firearms, such as pump shotguns, they got themselves a rate of fire and a capacity that was high enough to handle most unhappy social situations.
While a mag fed shotgun, firing from a 20-round drum, semiauto and full auto, with carbon fiber, titanium, and a holographic sight, would be more modern and high tech than a pump shotgun per se, there are very few situations that it will handle better than a pump shotgun. I mean, hell, most of the time a Winchester 97 would do the job just as well as the most modern shotgun.
In other words, Skunky, the M3 may be more modern and tactical a plain-jane 870, but the advantage it offers (semiauto fire) is a very small one, especially given the cost difference between it and an 870/1300/590A1.
It really says something about firearms development. A lot of the things we cooked up a hundred years ago are plenty good for any unpleasant encounters. The weapons are better now, but people themselves are the same soft, chewy centers as ever.
Regardless, it's your money. The M3 certainly is bad*** looking, and you'll probably be the only one at your range to have one. So, if you dig it, go for it. But Skunky, I would've though that you'd dig the HK FABARM semiauto more. It's HK, after all, and it's available with a CARBON FIBER finish! And it's got the weaver rail for installing sights. I don't think the standard semiauto comes with a rear sights, though, for some reason.