Well, KQ7, the first edition, was only playable in Windows... The second version was packed with another interpreter, contained many fixes and both playable on DOS and Windows... This is probably not such a great difference, however, it's more than the usual differences between the same Sierra games...
And regarding the Hero's Quest/Quest for Glory... I think Hero's Quest, being sold for like a year, isn't that rare... I don't know how many copies have been sold, but it's likely that it sold more than the re-release, atleast that's usually the case... I was only aware of Hero's Quest for a long time and there are atleast two versions of it (one being 1.0)... Quest for Glory, due to the re-releases (collections, etc.) also didn't became rare (and perhaps the most popular?)...
But then again, what's the definition of 'rare'?
And what about the versions in other languages (Spanish, German, French, Italian)?