The 590A1 is Milspec and has all metal controls and a heavier barrel. Mine has the ghost ring and its pretty effective with slugs, kinda useless with every thing else imho. I kinda wish it didnt have the ghost ring, ive even thought about cutting the sight off and welding on a bead but its a matter of opinion. As far as interchangable Idk, i wolud assume that some parts would but im no gunsmith.
At home defense ranges, shooting buck shot, I'd say the ghost rings aren't worth the money. The bead will give you as less cluttered sight picture, and faster target acquisition.
If you want to shoot a ton of slugs at greater than 30 yards, and the ghostrings may be worth the extra $50.