The Express Home Defense 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.
A rougher blue job.
A less polished bore.
A two piece barrel. (not 100% sure about this)
Hardwood or synthetic stock, with a sporting-length fore end.
An 18", cylinder bore barrel, with a bead sight.
The 870 Police has:
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.
The Express is Remington's "Budget" gun designed to compete with the cheaper-to-make aluminum receiver-ed Mossberg and Winchester.
The Police is Remington's top-of-the-line Law Enforcement gun, made to a higher standard.
NOTE: Remington has announced that they have discontinued the Express locking safety button.