Get the one that feels best to you. The differences are relatively minor, and the 870 and 590 are the best of the widely available US made pumpguns. Buy a plain jane riot version of either, not some fancy name branded "special edition" version, and spend the extra $$$ on ammo and training.
Mindset - Skillset - Toolset. In that order!
Attitude and skill will get you through times of no gear, better than gear will get you through times of no attitude and no skill.