Differences in meaning
     
     "I'm going to do X": 
     	I'm revealing to you the plan, or intention
     	in my mind to do X.
     
     "I'm doing X":
     	I'm telling you about a process (on-going, in progress)
     	which has a certain time frame and in which I am involved
     
     This is a fine distinction and if you chose the "wrong" alternative
     no-one is going to think it is a big grammar mistake.