Firuza,
I also do not know if there are any major differences between the 1998 and 2001 NBDE Part I Dental Decks. I have the 2001 version and the only things that I can tell you is that there were printing mistakes in the 2001 version. The corrections are listed on the Dental Deck's website so we can correct them. I'm assuming that if the 2001 version has printing mistakes, then I'm sure the 1998 can very well contain mistakes as well. As far as more or less flashcards in 2001 than 1998 version, I have no clue! I guess it's always good to have the latest version! Just my thoughts and things I know.
😎