They are quite different. To mean a fixed plan (usually an arrangement or commitment, with others involved), you use present continuous plus a future time. For a plan that you've thought about but perhaps haven't made any arrangements for yet, use "be going to (verb)". You can also use this form to make a future prediction based on what you see in the present. There are a few other future forms we use in English (aside from "will" and other modals), so don't make the mistake of assuming that they're interchangeable.