I like the feeling south to north, feels like the ride is slowly building to a crescendo, starting with the gentle flat stuff through the farmlands and 'city limits' and gradually getting rougher and steeper.
Having done a lot of stuff in the highlands now, I would agree that there is better riding, but not that there is a better single A-B 150km route with the same contrasts in history, scenery and riding, plus I haven't actually ridden many descents more enjoyable than some of those on the way.
I think I enjoy it for the same reasons I enjoy the road bike, a real feeling of 'going somewhere' which is a bit different from the usual riding around in circles 😀