Tip pressure seems to be the defining difference between the two. The NRG builds more tip pressure as the water-break moves forward on the tip. But despite the high levels of tip pressure on tap, D3's brilliant design ramps up this tip pressure quite predictably, making it a useful, manageable tool—assuming, as with all skis, you have the setup right.
Here’s where I ended up after brief stints on each ski. I didn't get to super-fine-tune the setup on the ARC, but this was still a good setup. These NRG numbers, on the other hand, are fine-tuned and deliver a hard-carving setup:66” NRG
.785 (head)66” ARC
The binding position I settled on with the NRG is only slightly forward of stock. But it did an excellent job of restraining exposure to the tip-grabs and spin-outs usually associated with high tip pressures. This binding location, however, is heavily dependent on the associated changes to the fin settings. Just moving the bindings forward on the stock fin settings would hobble this ski's incredible ability to rip tight, hard-carving turns.
Since it’s tip pressure that differentiates these two skis, how should a skier choose between the two? Demo skiing is always a good answer, and perhaps it’s the best answer. But here are some considerations that may help.
The NRG is designed to work well for skiers who tend to ride through the pre-turn favoring the middle or back of the ski with the water breaking on the tip somewhere just ahead of the front toes. This makes it an easy-turning ski for long to mid line length skiers, but it also makes it a remarkably hard-turning ski for advanced skiers who can harness the NRG’s elevated tip pressure opportunities. This includes those hard-slammed backsiding turns at the ball favored by some super aggressive skiers (for this, move the bindings back ≈1/8" from my carving setup listed above). It also means this ski has a huge set of brakes for shortliners capable of riding a deep tip into the ball.
The ARC, by comparison, allows the skier to engage more of the front of the ski with less fear of tip pressure getting out of control. Using more of the ski favors increased acceleration and helps to maintain more speed through the turn, requiring less acceleration overall. This too is good for all levels of skiers favoring those who prefer more acceleration with less work, and reasonably tight, speed-preserving turns.
Both skis ride deeper than other “faster” skis. This deep ride is a big part of the cherished D3 stability. A deep ride requires a little more effort from the skier, but with stability comes confidence. And confidence is a key ingredient to winning performances—so “slow” skis are highly competitive skis. Another benefit to this deep ride is its resistance to blowing-out when scrambling gets frantic. If you are a skier who regularly dunks the whole tip into the water in search of more tip pressure, often ending in a blown tail, the ARC will most likely keep the tail in the game, and the NRG will absolutely dig in and explode out of the turn—if you have the strength and form to cash the tip-pressure checks this ski can write.
... Your ski should be your dance partner, not a wrestling opponent