I've had lots of both. Currently, have 3 500's and only 1 870 (and a couple of Win. 1200's which are a great in-between arm).
Are you going to shoot the hell out of it? Get an 870, you'll enjoy the quality difference. Defense/survival on a budget? The 500 will never let you down, and you can spend the $ you save on extra ammo.
Thanks. Defense/survival in case of TEOTWAWKI. Hopefully, it stays in a gun safe with a ton of ammo, oatmeal, and peanut butter.