The Police gun is better built. With some exception, the basic parts are the same on all the 870's.
Here's a list of the differences between the 870 models:
The Express is a "bottom of the line" budget gun, intended to sell at a price near that of the much cheaper to make cast aluminum and stamped Mossberg and Winchester.
Since the 870 Express still uses a forged and milled steel receiver and heavy duty fabricated parts, costs had to be cut, and what got cut was finish, and the addition of some plastic and MIM parts.
Here's the actual differences:
The Express Home Defense Model has:
A plastic trigger group.
The dimples in the mag tube and the new style plastic magazine retention system.
A rougher finish inside and outside, burrs left in place.
A less polished bore.
A rough blued finish.
A two piece barrel. (not 100% sure about this)
Barrel is Cylinder Bore.
Hardwood or synthetic stock, with a sporting-length fore end.
An 18", cylinder bore barrel, with a bead sight.
Some Metal Injection Molded (MIM) parts, like the extractor. (This may have changed recently).
Due to the long fore end and the dimples in the magazine tube, the gun won't accept longer side saddles or magazine extension without alterations.
NOTE: The new extended magazine Express does NOT have the magazine tube dimples.
The Wingmaster has:
An aluminum trigger group.
The old style magazine retention system, no dimples.
A much smoother finish inside and out, no burrs.
Gun receives hand finishing to insure top quality.
A top quality polished blue finish.
A chrome plated bolt.
A polished bore.
A one piece barrel.
Fine walnut stock with the famous "Bowling Pin" finish, with good quality checkering.
No use of MIM parts, the extractor is milled.
Will accept any and all accessories.
The 870 Police has:
An aluminum trigger group.
The old style magazine retention system, no dimples.
A much smoother finish inside and out, no burrs.
Police guns receive hand finishing to insure top quality.
A military-grade parkerized finish.
A polished bore.
A one piece barrel.
Barrel is Improved Cylinder.
Walnut or synthetic stock, with a short police-length fore end.
Many choices in different stocks, including Speed-feed, and others.
Has the new "R3" super recoil pad.
18" to 20" barrel, with a wide choice in sights, including rifle, ghost ring, and luminous.
Heavy-duty magazine spring.
Heavy-duty trigger-sear spring.
Sling swivel mounts.
No use of MIM parts, the extractor is milled.
Will accept any and all accessories.