You can use both forms to talk about future arrangements. The difference is that 'I'm travelling' is for a more fixed arrangement. If you say 'I'm travelling', this means that you have booked your ticket, made your reservations and everything is confirmed. 'I'm going to travel' might mean that you have a plan, but you haven't booked anything yet. It is less certain than 'I'm travelling'.

Strictly speaking, one is happening right now -- "I'm traveling", and the other is a plan, something that will occur - "I'm going to travel". But in reality, "I'm traveling" is often used to talk about plans also, usually ones that are definite. and in the near future.