Truthfully, they are somewhat better. They almost always have bonus tracks. I'd assume the Minotaur is Murray, the lovable mutated Malacovian giant, and there are a few albums with him on there. I'm not sure which albums you're thinking. But check the Metal-Archives for the set (metal-archives.com/band.php?id=163). From what you've said though, 4 Japanese Dio cd's in a box (I'd guess they're new) for ninety bucks seems like a good deal to me. Even good used condition sounds pretty good. Imports are really pricey, after all.
About sound quality, they're about the same. I can't here a difference and I'm die-hard Dio. Unfortunately, I'm not Japanese so I can't tell which albums they are.
Yeah, hope that lengthy thing helps. If not, post a response and maybe I can be of more help.