What would make you prefer a Remington 870 over a Mossberg 500? For me, the two were equal — similar in price, reliability, and customizability — except that the Mossberg was a better physical fit for me. I haven't had reason to regret my choice.
The 870 has a smoother action and the position of the safety is a better fit for pistol grip configurations. The 500 is generally considered more reliable and the safety is in a better position for non-pistol grip configurations.