I recommend Mossberg and here are the exact reasons why I do so:
There are probably more threads on this debate than any other topic... BY FAR. With many folks throwing in by simply stating the gun model they shoot, without offering much, if any, objective reasoning.
I can't resist throwing in my view (and I'll do my best to include as much objective reasoning as I can.
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Both are great guns... either is a solid choice. I've been shooting Mossberg 500 and 590 for years and they are tried, true, solid, and totally reliable. I have also just ordered an 870 Police, which I'm sure I will enjoy using for many years to come. But there are some very noteworthy differences between the Remingtons and the Mossbergs as far as controls and operation goes.
Comes down to this for me:
1) I prefer the carrier-up mode design of the Mossberg for really easy loading... no need to bother with pushing the carrier out of the way to access the tube for loading/topping off while shooting. One can get used to dealing with the carrier being in the way (870), but having the carrier out of the way (500/590) is much nicer - especially when topping off under duress.
2) I prefer the tang mounted safety of the 500/590... you can see it VERY quickly and easily, access it with either hand for left or right hand shooting, and you can operate the safety while your trigger finger is ready to shoot, without changing the position of your shooting hand! No need to use your trigger finger for anything BUT firing the gun. Seeing the safety position on the 870 is more difficult and operating it takes more hand movement.
3) I prefer the slide release lever on the 500/590... a slight shift of the middle finger releases the slide without having to change or shift the position of the hand and without moving your trigger finger from ready/fire position. With the 870, the shooting hand must be moved from firing position to reach under and over to the left side.
4) I prefer having the double extractors (on on each side of the bolt) with 500/590 vs. a single extractor on the 870. Two is probably better than one.
5) I like the standard 9-round capacity of the 20" Mossberg 590. For a defense shotgun, nine before reloading again is very nice to have.
And Mossberg has a new 500 Persuader 20“ 8-shot available as well, which has an extended magazine tube. The Mossberg 18.5” 500 Persuader is a six-shot model and feels very nimble and quick handling due to less out front length and weight. You can't extend the mag on the 18.5" 500 due to how the barrel attaches to the mag tube. Remington has hampered the ability to extend the mag tube on the 18" Express by putting dimples in the mag tube (also cheapens their cost of production on this gun.) It is possible to get around the dimples by drilling or dremeling or pounding them out, thereby increasing the capacity from five to seven rounds on the 18" Express.
6) I like the availability of full length stock & pistol grip combos on the Mossbergs. While shooting with a pistol grip involves a fair bit of compromise... it does offer a form factor which is extremely appealing for certain circumstances! And if the BATF caps down on shotgun mods (they have recently reclassified a shotgun with a pistol grip as a PISTOL - no longer a shotgun)... if you bought a factory made shotgun which came with a pistol grip originally from the factory, it could be argued that you haven't materially altered the form of the shotgun if you choose to attach different pistol grips, or birds head grip options. Legal grey area here, it appears, but something worth considering anyway.
The steel receiver of the 870 is arguably more durable than the aluminum receiver on the 500/590, BUT I've never seen nor heard of any issues, nor shortened life associated with the receiver on the 500/590 and I doubt that the average shooter would ever experience a problem with the receiver being aluminum vs. steel. This materials difference between the gun's receivers is virtually negligible when comparing the guns vs. the more significant differences in operating controls. I also like that the Mossberg's aluminum receiver makes for a slightly lighter weight gun.
Quality of manufacture... I think all Mossbergs are of similar build quality and fit and finishing. They are solid, reliable, durable and not fancy.
Remington’s Wingmaster line is of higher quality in the fit and finishing, but is MUCH MUCH more expensive than Mossberg - perhaps two to three times the price!
The Remington Police line has perhaps a lesser level of finishing and smoothness than the Wingmaster, but is of higher quality fit and finishing than the Remington Express.
The new Remington Express line really has cut corners to make the gun as inexpensive to manufacture as possible... this in an attempt to compete with the very attractively priced Mossberg, which is quite popular! I think perhaps Mossberg builds a better lower-priced shotgun than Remington does.
If you want higher grade polish & finish, be prepared to spend a lot more money and go for a Wingmaster or Police. In the lower price range, Mossberg competes quite favorably, perhaps superiorly, to the Remington Express line. Big nod to the Mossberg from me. But both are competent shotguns.
As for overall durability and lifespan... if the gun is a personally owned, home defense weapon, of an ordinary citizen, I believe it is extremely unlikely that one would encounter any noteworthy limitation in durability or longevity. We're not talking about a service issued weapon that is fired a bazillion times by hundreds of different people, day-in and day-out, under the most difficult, abusive conditions imaginable, for years and years-on-end. So, I don't really see this as being terribly important as a selection criterion for a personally owned home defense weapon.
Though my preference is for Mossberg mostly due to the location and operation of the controls and the lifter-out-of-the-way for loading, no doubt one can become accustomed to either platform and both can be considered equally reliable. I would feel very well covered having either a 500/590 or an 870 as a personal defense shotgun.