I'd rather have the early original-- I'd like to see the colors Leonardo intended, though I suppose he was such a geek that he might have tweaked them to allow for some fading.

Paint or Pixel: The Digital Divide in Illustration Art has more than a little (and more than I read) about what collecting means when some art is wholly or partially digital. Some artists sell a copy of the process by which the art was created, and some make a copy in paint of the digital original.

Strange but true: making digital art is more physically wearing than using paintbrushes and pens and such.

Note: the book isn't about illustration in general, it's about fantasy and science fiction illustration in particular.