Please read this conversation:
Boss: "someone has to go to Vietnam next week"
Tom: "I will (go there)"
Kate: "Oh I'm going to go there next week."

I think "I will" suggests the speaker decided just after the boss's request while "I'm going to go there" suggests the speaker had already decided the trip before the boss's request.

My question is : Is it OK to say "I'm going there" in this conversation? If so, could you explain the difference between the three sentences.

Question

Please read this conversation:
Boss: "someone has to go to Vietnam next week"
Tom: "I will (go there)"
Kate: "Oh I'm going to go there next week."

I think "I will" suggests the speaker decided just after the boss's request while "I'm going to go there" suggests the speaker had already decided the trip before the boss's request.

My question is : Is it OK to say "I'm going there" in this conversation? If so, could you explain the difference between the three sentences.

Answer

"Will" is a verb, but "going" is only an adjective. Verbs act, but adjectives only describe. "I'm going there" describes the person as being in the category of all people who are going to Vietnam. It has no force. It just says "I'm on that list".

"I will go there" has force. It asserts either willfulness, willingness, or determination. It asserts control. It makes a statement. It has power.