I'd get the 870 but 3" only and then buy a rifled barrel for it.
In my opinion, there is no reason for a 3.5" shotgun shell. In fact, I goose hunted with a guy who shot a 870 magnum with 3.5" and he could not hit the broad side of a barn because of the recoil. Also, he would short stroke the action all the time because the stroke is longer.
I have owned a Mossberg 500 and I sold it, I hated it. If I were you I would buy a Beretta Outlander new or used whatever you can afford. Shoot buck shot for deer if you like and it can take any bird out there. Pumps can shoot rifled slugs 50-75 yards accurately but some semi-autos can't shoot slugs.
If you are set on a pump, I recommend the Winchester SxP, Benelli Nova, and Weatherby Pump. Remington 870 is ok for if you want multiple barrels but I have seen an 870 express literally blow up in the field. Not in a dangerous was, a guy just shot it and pieces went everywhere, spring must of been loose or something, pretty funny though. Look for Remington Wingmaster if you go the Remington route.