In my opinion, don't get sucked into the "tactical" marketing BS from the respective firearms manufacturers.
Decide what features you want.
20 posts in here on CGN, so I'll assume you're looking at pictures and wanting a gun that looks cool/badass. I was much the same when I started out, but have refined my tastes somewhat.
Full disclosure: I have a Remington 870 Express Tactical like the one pictured.
First thing I'll say is, Ghost Ring Sights are USELESS. You're not going to be able to see them at night, and during the day you're going to take longer to get an accurate shot off on a zombie (I'm assuming that's the badass reason behind your search for a badass Tac Shotgun). Skeet shooters use a bead for a reason: you can get accurate shots off very quickly with them. I can shoot skeet without even really aiming at this point. I take the rail off my shotgun and the front post oft, and aim down my barrel/receiver.
For "OH COOL, THIS SHOTGUN IS BAD ASS" comments from friends: Ghost ring sights and a top rail is awesome.
For practical use in a SHTF scenario, I think they suck ass.
Now, regarding your choice of firearm.
I personally like Mossberg more than Remington because of the safety and slide release locations. Far superior. The Mossy's shell lifter also stays up and out of the way, which I like, too.
If I were you, I'd get an 8 shot Mossberg 500 or even a 590, foregoing the Ghost Ring Sights (GRS). GRS sucks.
Benelli's are nice shotguns, too, but I haven't owned one. I don't really have a preference for them one way or another because I haven't played with one. But I will note that they are popular in 3-gunning matches down south.
Also, I'd always go for the maximum capacity in a tactical type shotgun. This means a 6-7 round tube. Shotguns are limited in capacity as is, so I personally want as many rounds on/in the shotgun as I can get. No point in handicapping yourself with a 4-5 round tube.
[EDIT]
IMO, the SPX is a waste of money.
When considering a Mossberg, keep in mind that if you're getting the pistol grip stock, you're losing/gimping the two BEST FEATURES of the Mossberg platform. You're not going to have your thumb anywhere near the safety, and you're going to have to reach like a bastard to hit that slide release. The pistol grip stocked Mossbergs are the sure mark of a total dummy. People will be able to point you out as a dumb ass from 1000 yards away.
8-shot Mossberg 500 with 12" LOP Hogue stock for the win.
For the record, I LOVE LOVE LOVE my Remington.
But the more I play with my guns, the more picky I get about certain things, and the controls on a Mossberg just seem to turn me on the most. I'm actually currently looking for a cheap 8-shot Mossberg 500.
This 590 is an example of "all you need" with the exception of an 8 shell side saddle.