I can't speak to the 870, haven't used one in awhile, but I fixed a bunch of our 500s, like 150 of them, with Wolff springs and Vang stainless followers.
The factory springs suck, crap out way too early and tend to rust, the factory followers are a bit flimsy and tend to stick in the tube with any amount of dust bunnies, or as it turns out rust specs from the springs. No abuse at work, just our humid summers here.
I can highly recommend the Wolff/Vang combo