People have debated that issue for years. I am not that interested / vested in it but the MPFW tends to be more vocal about it hence the designation of MPFW as the first appearance. IIRC, the proof comes from slight difference in the art between the two appearances.
Loc: The Great Northwest
I remember something about the "coming next week" box at the end of the story being blank in the MC #1. I think VintageComics (Roy) posted the pages to the Subby story from MPFW a few years ago. Very cool stuff.