The difference is $30 in parts and parkerizing and furniture.
There is one Remington 870, and they all differ in the finish and furniture and in the case of Police 870P, about $30 in parts ( i think its 4 ? parts?).
I have an 870 Tactical which is an Express model, the finish sucks lemons, I have to admit. I am thinking about the Wilson Combat upgrade but regardless, it's about $30 in parts and parkerized vs cheap-aced bluing called 'matte powercoat' in the Express and really nice deep shiny rich bluing in the Wingmasters and thats it. (Oh and furniture- different stock and forends.)