[@TheUnknowable] A Cleric chooses to worship by choice, a cleric also doesn't have to have his magic given to him by a god, it can be from another source, a cleric can literally not worship a god, as long as they have some kind of faith in something, also Clerics have considerably more freedom than Warlocks, who more often than not are just the slave of some eldritch deity, Also Clerics tend to wear big heavy suits of armor, Warlocks don't do that.
Warlocks do not choose, their faith is thrusted upon them whenever they want it or not, their source of power must come from a Demon, Cthulhu or Fairy. A Warlock can hate his own deity as much as he wants, but Clerics can't