The PHB states that a trident deals d6 piercing damage (d8 with two hands) and spear deals d6 piercing damage (d8 with two hands).
Trident can be thrown 20'-60' and spear can be thrown 20'-60'.
Trident weighs 4lb and spear weighs 3lb (more can be carried).
Spear costs 1gp and trident costs five times more (more can be bought).
So speaking logically one would not choose a trident over 5 spears. In my campaign never have I seen a man with a trident.