You know, that's a good example, you have a good point. In some contexts they are slightly different. I'll take a wack at describing the difference though: "I'll go to work tomorrow" sounds like making a decision out loud, in the moment, while "I'm going to go to work tomorrow" just sounds like communicating intentions, regardless of when the decision was made or whether they were the ones to make it. In this context "I'm going to go" can substitute "I'll go", but the reverse might sound weird. So I concede but only a little