@Stevie Boy
Nice question, one that many people seem to have, especially those of us "across the pond" where it's not so easy to demo the two.
Terry Winter initially suggested the NRG for my weight/height/speed (200lbs/6ft/34mph), and said it's more forgiving, but after watching a video of my skiing said the ION S would probably suit me better because it would help on my offside, where I have some bad habits.
Interestingly, on the split screen, it looks to me like he finishes his turns slightly better on the NRG, but as mentioned above, he can ride anything. It would be more useful to see that comparison by less experienced skiers.
Did you end up getting one?