That 870 in your link isn't a bad gun, but given the option of a new express based gun for 328 or a used police gun for 270, I'll take the used police gun.
Police trade in pro's:
You get the rifle sights (a real plus for slug shooting)
You get the Speedfeed stocks (nice shell storage and much more solid feeling than the stock Remington Synthetics
You get the police based action with metal triggerplate and chromed bolt.
You get the aforementioned short forend (though for shorter statured shooters the longer forend can be a plus).
Police trade in cons (vs. the express gun you linked):
You have to buy a magazine extension if you want one (fairly cheap on the used market for a Remington factory).
Finish will be more "well loved" but if you use yours it will be too...
That's all I got....
NC Hunter's Education Instructor
Member (What's PCCA you ask? <- Check the link)