I just purchased the blackhawk collapsible stock for my 500. My shotgun came with the pistol grip as well, but I wanted the option of the stock. It is made by Knoxx and it pretty good for my use. I have yet to shoot with it though. I found it for $70 with foregrip at Walmart.
I purchased my mossberg 500 with a pistol grip also, I replaced it with a "ATI top folding stock" I really enjoy it, I like having pistol grip when you want or else a stock. It's lightweight but only because it is made entirely of plastic, but that is why it is so affordable. The only issue is that when friends and family shoot it they find the stock a little long for people with average to small arm spans. I have long arms being over 6' tall and it works great for me.
I have the knox recoil reduction stock on this cut down 870 12ga, even with 3" slugs or buckshot, it does help reduce felt recoil, it used to uppercut me in the cheek when I first started shooting it, but now that I know how to shoot it properly it helps quite a bit!