3.5" isn't worth getting, though. It's a more complicated mechanism and rougher to work the action.
A regular 3" 870 is really reliable, simple and easy to clean, and it will shoot anything that you really want to shoot. (3.5" doesn't offer much in steel shot since range is limited, and if you use Hevi-Shot for geese or something you don't need the longer shell. You don't want to use 3.5" buckshot loads for HD. Too much recoil and blast. I'd rather use 2 3/4" and have the option of a quicker followup shot.)