Faris is right, those are the differences.
I'm not sure that these differences are enough to warrant paying the price difference between a 870 Express HD and a 870 Police.
When I was at Scattergun, we built a whole lot more Expresses into riot configuration than we did Police models. Why? The differences are not a factor when you trick out a shotgun. You just need a solid platform to start with.
We replaced the stocks with fiberglass stocks.
We used our own springs.
We removed the detents in the mag tubes.
Granted, this was before the safety lock, but we used the plastic trigger housing with no negative response.
At the time (4 years ago), a Police Magnum was $200 higher than a Express. Almost double. I'm sure that's still the case.
I encourage my customers to look for police trade in 870s. The last one I built up, the guy paid $150 for the base gun. I am starting on a 1100 now that the owner paid $275 for on Gunbroker.com. When he gets it back, it will be as new.
Save some money and buy a used 870 Wingmaster with a plain barrel. Put the money you save toward a conversion.