Although the FN it is a nice shotgun I went with the 870 Police, with a full length Choate Mark 5 Pistol Grip Stock, factory tritium nite sites, a Wilson extended mag and a Wilson Side Saddle. My total cost was less than the FN and I really like it my combination.
Other than cost, my main complaint on the FN was the stock. I shot an FN at our local range and while the FN telestock is fine on an AR, it sucks on a shotgun when you routinely fire 3" slugs and I wanted a pistol gripped stock that would help manage recoil. The Mark 5 fits the ticket and can be adjusted to fit your body (and clothes for winter hunting).
I think the advantage to the FN is the rail (KACs for the 870 are $ and hard to find) and the ability to have a BUIS. It was probably designed for people familar with the AR system, so it has the perk, but it wasn't important to me.
Good luck with the decision.