I have a couple of 870 Tactical sg's and they work fine. There are some differences in the operation of the 870 and the Mossberg 590's that probably should be addressed in order for you to make a somewhat informed decision. I only mention those two because they are the primary ones selected for tactical work.
Mossberg pluses: (just my option)
1. Extra long magazine is stock on 590s. I think it holds more than the extended mag model or after market extension available for the 870,
2. Aluminum receiver making it faster to bring into play, and much easier to hold for extended period especially at 'low ready'.
Take a 2-4 Tactical Shotgun class and you will appreciate the difference.
3. Factory installed ghost ring sight models are a bargain.
4. Tang safety easy to find.
5. Shell release easy to use to unload tube w/o racking them through the chamber.
6. Carrier is out of the way when loading giving better reload times.
7. A little cheaper in price (remember that it has the sights, too)
870:
1. Stable proven, reliable design. (Vang Comp builds a lot of his comed 870 using the Express models and he says they are not a problem).
2. Needs big head safety added, still not as easy to use as a tang safety.
3. Ghost rings run a couple of hundred or so.
4. My 10-10.5# 870's allow me to shoot 300-500 tactical rounds in 2 days w/o a flinch or sore shoulder or cheek. Those weights include 13 rounds on the 19"gun, 14 on the 21" cantilevered, both hve RedDot scopes for my old eyes, slings, and side saddle shell holders.
The head shotgun instructor at FrontSight uses a Mossberg and runs it faster than his Benelli. Reports no problems so apparantly Mossberg has cleaned up their act since 1960(?) and the 500s. The head TopGun instructor recommends the Mossberg as the best tactical pump. You probaly 'read' that I am partial to some of the features of the Mossberg and that is correct, but I am not getting rid of my 100% dependable 870's, but would try a Mossberg if I was getting another Tactical shotgun. My shooting buddy mentioned after our last 2 day class that mine was the only gun there that ran flawlessly both days. That incuded a couple other 870's, Benellis, Winchester 1300 (the instructor called it 'a really good sporting goods gun' not a good tactical gun). One of the problem 870's was highly modified and the mag extension tube was not machined properly and it fell off a couple of times, then he used a borrowed gun. Handload ammo may caused some of the problems, I used factory as required by the class. Good luck on youir selection. Jim