V + -는 거야 used for explaining the action about which the listener has some information. V + -고 있어/다 used for explaining the action about which the listener has no information. For example, Q: 지금 뭐 하는 거야? : What is that you are doing now? (The speaker saw some actions or heard some noises.) A: 책 읽는 거야. : What I'm doing now is reading a book. Q: 지금 뭐 하고 있어? : What are doing now? (The speaker has no information) A: 책 읽고 있어. : I'm reading a book. 우리 지금 어디에 가는 거예요? Where are we going now?(The speaker saw some actions or has some information.) 우리 지금 어디에 가요? Where are we going now? (The speaker has no information)

It's all about tone. You could say "박희섭(Heesob Park)" and "hyun" are both correct. When you use "우리 지금 어디에 가는 거예요?" normally, then it would be same with “우리 지금 어디에 가요?”. But when you say the former with some irritating tone then it would mean the one "박희섭(Heesob Park)" elaborated.

just the way you say is different but the meaning is exactly same.