When are Seals vs Green Berets used?
Doctrinally, SEALS are a maritime force, Army Special Forces lean toward land based guerilla warfare, raising indigenous armies, etc.
In the real world, it gets very murky. For example, Army Special Forces have more qualified combat divers than the SEALS, and the SEALS have more qualified free-fall parachutists than the Green Berets. Basically, they all try to be jacks of all trades, and whoever has the most influence at the moment gets the mission. This translates into more $$ at budget time.