a lot of the debate is subjective, but here are a few of the basic differences:
- the 870 has its safety located on the trigger guard (similar to the 10/22). The 500's safety is on the top of the gun at the rear of the receiver. As a lefty, I prefer the 500's safety location as it is more ambidextrous, but that is mostly a matter of personal preference.
- standard 870s have 5-shot capacity. Standard 500s have 6-shot capacity. Probably not a big consideration, but some people prefer more capacity for defense weapons.
- The guns weigh about the same, but I believe the 870 has a steel receiver while the 500 receiver is made of aluminum. As a result, the 870 may be slightly more "center heavy" which is generally considered a good thing in wingshooting. May not be relevant for defense.
- 500s are a bit cheaper and extra barrels are significantly cheaper if you, for example, wanted a short barrel for defense, a long barrel for clay targets and hunting, and a slug barrel for deer. Also, 500's come with three choke tubes. 870s come with one (buying extra's will be about $15 on the low end). That said, the difference in cost won't seem like much down the road if you really like the 870 better.