There really isn't a difference. They just keep making the same cake and redecorating it. Todays 883 sportster is the same 40 Hp as the 1930 900 knucklehead. Nothing really changes except the marketing of a modern day antique. They are all the same gutless overpriced fashion accessory marketed to people who know no better.
Well from around 1903 they were dinosaurous ancient, then around 1933 they were dinosaurous knuckle, then around 1955 they were dinosaurous panhead, then around 1966 they were dinosaurous shovel, then in 1984 they were dinosaurous evolution, and in 1999 they introduced dinosaurous twin cam.
Actually they have only slightly improved since the almighty rode a dinosaur.