Personally, if I was going to spend that kind of money, I'd be buying a Remington 870 Police shotgun.
The Mossberg uses a cast aluminum receiver, and the 870 uses forged and milled steel.
95% of all law enforcement in America use the 870, and the reason is, no shotgun is as durable, reliable, and tough as the 870.
The 870 has a 50 year, 7 million gun reputation for this, and no other gun sees as much abuse as the police give them and still work.
These guns just DON'T break, wear out, or give trouble.
The best estimate of the service life of the 870 is 250,000 rounds. That's a Quarter of a Million shotgun shells.
For a look at what Remington offers:
http://www.remingtonle.com
To buy a Police gun, you need to find a dealer who will special order one from a Remington Law Enforcement Wholesaler.
If money is an issue, buy the 870 Express 18" model, and add whatever accessories as you want.
That's another BIG advantage of the 870: More accessories are made for it than all other brands combined.
The only "problem" with the pistol grip full stock pictured is, some people report that the rounds may tend to "bounce" out of the stock, or the springs may tend to jam up if not maintained carefully.
Whether the stock is "good" or "bad" is strictly a matter of person preference.....some people like the feel, and some don't.
Of the guns you're looking at, the 870 Express is the best "deal" money-wise, and you can modify it into anything you want.
If you want the "Cadillac" top-of-the-line gun, buy the 870 Police.