I started around the time the switch was made so I got the horse PHB, the white MM and the black DMG. They're the same in terms of actual game material, except that the black books correct errors in the earlier books. The black books have a higher page count as well, my PHB has triple columns, while my DMG has double columns, and a larger font. Interior art is different, the 1989 originals were black white and blue inside, while the 1995 were full-color. Layout of the 1995 edition is the same as was used in the PO books. Also note the MM is from around 1992 or 1993 originally, before that 2e used the Monstrous Compendium binder.
Also on the subject of 2e books: the Tome of Magic was later printed as a black cover with the original art on the book. The MM had a black cover version too, I don't remember if the black MM came later than the PHB and DMG (I think it did). I think one reason for this was that the old MM and ToM had the old 2e AD&D logo, while all black books used the updated red AD&D logo.
Around 1998 or so some of the books like the ToM and PO books were printed in softcover, but I don't think the three core books were ever printed in softcover.