I liked the song for song contrast idea that you did, I don't think I've ever seen it done before on MB, which is surprising really for what seems like an easy idea and great idea.
The problem with the Black Sabbath debut is that the b-side contains too much filler, apart from that the rest is a classic though.
The Helloween albums that you've listened to are the tip of the iceberg when it comes to power metal. You now have the European or American styles to enjoy. As for Helloween the only other album that comes near this in quality is Better Than Raw.