I like the 870 over the 500, no real reason why I just do. I really like my 20 gauge, got rid of my 835 after getting the 20, but if I was only going to shoot lead a 3" 870 12 gauge would be my choice.
If you don't care to throw coins at hevi13 or federal HW's a 20 gauge stoked with them is bad news. I figure ammo is the cheapest part of hunting so I will spring for the good stuff. Dead is dead, it's overkill, etc but it's my money and projectiles are the only thing that you have that makes contact with what you are killing, so I figure why be cheap on that part.