That's all cool and I totally respect what you are saying but Mystery of Time and Mindcrime are pretty much from the same genre. Just to clarify my post when I called Mindcrime one of the greatest albums of all time, I did mean in the genre of metal/hard rock music not all of music as I don't think it would be universally accepted to be the case. It isn't like trying to compare The Beatles Revolver album against Mindcrime etc..Where I am coming from is that if you love Avantasia, and I know that you also love Iron Maiden then I have a hard time understanding how you couldn't love Mindcrime.
My situation is probably different to yours, in that I heard Mindcrime pretty much on release day in 1988 and it basically changed my life. I appreciate that you aren't coming at Mindcrime in the same way, however I just feel that Mindcrime has a far superior depth of story, great lyrics, emotional impact, awesome vocals, guitar playing etc that The Mystery of Time doesn't have.
While they're in the same genre, generally speaking, I don't think they are all that similar stylistically. There's a different sound to those two albums. I also think Queensr˙che has a clearly different sound from Iron Maiden. And I just like Avantasia and Iron Maiden's sounds a lot better than Queensr˙che's.
The features you list for why you like Mindcrime better are totally valid reasons, but most of them are personal. For my own part, I don't love Tate's voice, even at his best. I think he's a talented singer, but the timbre of his voice isn't ever going to be my favorite. He might be more talented than, say, Tobias, but I'd rather listen to Tobias. And I'm not a big fan of the Mindcrime story. I don't tend to enjoy stories that focus on subjects like crime and insanity. So the story actually detracts from my enjoyment of the album.
It's hard for me to say more about the difference between these two albums without using classifications that make sense to me but may not make so much sense to others, but I'll give it a shot: I have a definite classification of two types of music that I call "warm" and "cold." I can't say exactly what makes something warm or cold, but I know it when I hear it. And, all things being equal, I vastly prefer warm music. Avantasia and Iron Maiden are both definitely warm. Queensr˙che is cold. So even though all three are metal with prog leanings, I consider them to have very different sounds.