It's really personal preference. The M14x1 LH is the traditional AK-47/AKM muzzle thread and the M24x1.5 RH is the traditional AK-74 muzzle thread. So depending on what type of muzzle device you want to put on your rifle, you'd pick one or the other.
Fun fact: The AK-74 barrel can be threaded with M14x1 LH, since the M24 thread is actually part of the front sight and not the barrel.