I have been wanting to buy a new Remington 870 Police at my local cop shop for a while, have been saving up for one. I can get an 870 Police model with extended tube (7 rounds) 18" cylinder bore, synthetic stock, parkerized for about $460. But I just found the 870 Express Magnum with identical specs at a chain sporting goods store for only $279! I do not think that the Police roll mark on the receiver is worth an extra $180. Are there any differences in these two versions other than the markings? I cannot see any, but I cannot compare the two side by side. Any thoughts? Any reason I should not save the $180 and go ahead and buy the Express?

When you say "870 Express Magnum", I'm assuming you mean it has a 3.5 inch chamber.  I would pass on the police model and go with the $279, or even better yet, a regular 870 Express with 3 inch chamber.   I see them for $199 here sometimes.  Then you can have fun accessorizing it with all that leftover money.

The 870 Express Magnum I am looking at is a 3" chamber. The specs are identical to the Police model. I want to know if there are any "hidden" differences not apparent in a visual inspection, such as internal part differences, etc. There is not much I would want to modify on the Express model, it is setup in a good configuration. I believe some people call this the "Home Defense" model, but it is marked and marketed as a normal Remington 870 Express.

The Police 870 has:

A smoother, better deburred finish inside and out. Overall the gun is smoother.

A high grade parkerized finish versus the black finish of the Express.

A real walnut stock vs. the "hard wood" of the Express.

A military/police length fore end vs. the Express' civilian long fore end.  The Express must be either shortened or replaced if you want a side saddle.

A heavy duty magazine spring.

A heavy duty trigger/sear spring.

The original magazine tube assembly vs the Express' "dimpled" assembly.  Mag extensions screw right on, with no modification necessary.

Better choices in sights and stock options.  The Express HD has a 18" bead sight barrel with a cylinder choke.  
The Police comes with a wide variety of sights, stock designs, magazines, and barrel lengths, with Improved cylinder choke.

The Police has a metal trigger guard assembly, the Express is plastic.

The Police has NO lockable safety button, the Express has one.

If you want the gun set up right out of the box, the Police is a better shotgun. Whether or not it is worth the extra money is somewhat debatable. You can take an Express and do pretty much the same things to it as you are able. The dimples are not a problem to remove, and if you are going to replace the stock/fore end with something like a Speed Feed set-up the walnut is just money spent for nothing. The plastic trigger guards have, so far as I know, caused no problems yet. (The "metal" guard of the other guns is only cast "pot metal" and it would not surprise me too much to find that the "polymer" guard is actually stronger)

I have both a Police Model and an older Express along with two Wingmasters (yep, I really like the 870) and the Express has been nothing but reliable. "Over the counter", however, the Police is a much better finished weapon.