870 and 500 are kings in the affordable pump arena. Can't go wrong with either one.
I went with the 500 for several reasons, in no particular order:
-cheaper than the Rem 870
-tang safety is a more logical location, imo, and means there's one less switch by the trigger guard
-slide release on the 870 is in front of the trigger guard; on the 500 it's behind the trigger guard, right where your middle finger can easily hit it - I think it's a better location than on the 870
-shell lifter on the 870 stays in the down position - the one time I shot a Remington (it was an 11-87 but same lifter as 870), my glove got caught in the lifter while inserting shells; when I took the glove off, I pinched my finger. No, not a huge deal, but still nowhere near as convenient as on the Mossberg where the lifter stays up.