As long as you add the time, it's clear that this is in the future and so you don't need "going to." Both of your sentences are 100% correct. However, your question should have been: "Is THERE a difference when I am talking about something THAT I have already arranged to do?"

Big difference! I am doing = now I am going to do = not doing yet, but will soon. (Or soon-ish... this is a future form, indicating there's a plan or process underway already.)

Я иду домой. = I'm going home. Я пойду домой. = I'm going to go home. I'm doing ... (doing it right now, in the present) I'm going to do ... (planning to do something in the future)