Mossberg 500 or Remington 870 for a first shotgun?

You can't go wrong with either — both are proven, affordable, and everywhere. The 500's safety is on the top tang (ambidextrous, intuitive); the 870's is behind the trigger. The 870 (especially older Wingmasters) has a slightly smoother action, but current Mossberg 500/590s are excellent and often cheaper. Handle both and buy the one whose controls feel natural to you.