Buy what feels best but if you do opt for an 870 I would not buy an express. I would buy one of the higher end models with the stronger internals. The Police or LE models for sure would be a solid buy. They are more $$$ but you get a better gun.
The Mossy 590A1 is a real solid choice and dollar for dollar a better scatter gun than the 870 IMO. Like I said, dollar for dollar. The 870 Police is probably what I would buy. You can be guaranteed there will always be an aftermarket for the police model.