Here's the current differences between the Remington 870 shotguns:
The Wingmaster and the Police are the same gun with different cosmetic finishes.
The Express is Remington's "budget" gun made to compete with Winchester and Mossberg's much cheaper to make, cast aluminum receivers and stamped internal guns.
Almost all 870 shotguns are now 3" Magnum models, and have been for some years.
All Remington shotguns have the same forged and milled steel receiver.
The Wingmaster Sporting gun has:
Aluminum trigger group.
A smooth, commercial grade polished blue finish.
Very smooth, no burrs inside or out.
A smooth, polished bore.
Milled extractor.
Walnut stock with better checkering, and Remington's "Bowling pin finish".
A chrome plated bolt.
Old style magazine retention assembly. (No dimples"
One piece barrel.
The 870 Police is basically a dull finish Wingmaster.
An aluminum trigger group.
The old style magazine retention system.
A much smoother finish inside and out.
A military-grade parkerized finish.
A polished bore.
A one piece barrel.
Walnut or synthetic stock, with a short police-length fore end.
Choices in different stocks, including Speed-feed, and others.
18" to 20" improved cylinder 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 lockable safety.
No use of MIM parts, the extractor is milled.
The Express Model has:
A plastic trigger group.
The dimples in the mag tube and the new style magazine retention system.
A rougher finish inside and outside, with burrs left in place.
A rougher blue job.
A less polished bore.
A two piece barrel. (not 100% sure about this)
Hardwood or synthetic stock.
Some Metal Injection Molded (MIM) parts, like the extractor.
NOTE: It's possible Remington has discontinued the MIM extractor due to breakage problems.
Also, It's not clear at this point if Remington is still using the "J" lock lockable safety button on the Express and Wingmaster guns.