Arts, a few things about this well chewed subject. I'll save you a few hours in the files and hit the high points.....
Any of the Big Four(Remington 870, Winchester 1200/1300, Mossberg 500-590 and the Ithaca 37) are SUPERB close range defensive weapons in trained, responsible hands.
Training is much, much more crucial than brand, accessories, mods or what the latest gun of choice is down at the local range/shop.
Shotguns are MOST effective when operated from the shoulder and aimed. Spread is miniscule and choke unimportant at typical ranges. The load will hit as a solid or semi solid projectile and dump an incredible amount of energy into the target. THIS is the big reason to employ a shotgun, high percentage (99%plus) one shot stops.
As for cost, police turnins(usually 870s) are the best bargain out there.Oft beatup on the outside, they're nearly new inside most of the time. Two of my 4 12 gauge 870s started as agency weapons.
Get the shotgun that FEELS best from that short list of the Big Four. Shoot it until it feels like a body part instead of a tool. At that point, brand is a moot issue, effectiveness is a given....
HTH....