Every time our class meets, my Modern Irish Literature professor gives us homework. All we have to do is write a long paragraph about whatever prompt question she gives us. It's very low-stakes - it's just supposed to be free-association. I guess to find out what we "really think"? Hahaha. This week, the question is: "Do you believe there is a difference between 'memoir' and 'autobiography'?" I can't tell if my response is more like raw talent or a serial-killer's trippy hallucination. So you (and future me) tell me.
I do believe there's a difference between "memoir" and "autobiography". It has to do with the connotation of both words - "biography" implies a strictly factual, dry account of a person's life; "memoir" implies a more impressionistic story.
As for whether Angela's Ashes is fiction, memoir, or autobiography, I think it has to do with the author's intent. Did McCourt set out to tell us a story, inspired by his life? Or are we supposed to believe this is a historical account? Can historical account (biography) be art? Did McCourt want to make art?
Hard to tell, of course, since he's dead now. But surely someone must have asked him, and if he was as much a jerk as everyone said, he probably said something. And I suspect that if he did answer, it would have been something along the lines of "Nope. It's all true..." albeit with a smirk.