Well, it depends. For occasional re-dressing (that all hones need) the sandpaper works OK and isn't hard to use. On the other hand, if I had used sandpaper for the initial lapping of my Norton I would have used so many sheets of it that I probably would have been about half way to the cost of a DMT.:gaah:
I usually use my lapping plate hand held under some running water which would not be possible with sandpaper (as far as I know). So for me, there is a difference. This is one of those YMMV things.