they both are great, ive skiied both, to me the hellbents feel more like a normal ski with rocker, making it super easy for pow.  and the ep's feel like surfboards, i think the early taper, the softness, the light weight, and the little extra width makes it feel like that,  in powder, it feels like your skiing on top of the snow where hellbents feel like your in it.  with ep's i could actually get the tails to kinda slide out, and sorta slide the pow a little sideways, which was really really cool.  hellbents feel more traditional.
the positive with the softness of the ep's is that you can butter in pow super easy, pop out of powder super easy, and you hardly have to land backseat at all on cliffs of pretty much any size, you can land with your skis even with the slope in super deep powder, and the soft tips seem to always stay above, the bad is that you cant land backseat, my first couple tries i was landing backseat and riding away in a wheelie or washing out, once you realize that you wont instantly nose dive if you land flat then its awesome.
hellbents were similar in that way, i was a little more sketched out landing flat with the slope, sometimes it seemed like they were ready to catch and nose dive, but with the hellbents you actually can land backseat and usually it pop's you back up, where with ep's you would stay in that position and have to muscle yourself back to normal hellbents still seemed to "wheelie" when you got too backseat occasionally but definatly not as much.
overall i think the ep's were a bit more fun for powder, they were suprisingly easy to charge steeps with too, you just have to trust the skis and ski with a somewhat normal stance.
in park i think i liked them both about the same. i LOVED the hellbents on jumps, where ep's are still super fun, but the extra width and softness makes it a little akward on bigger stuff.for smaller stuff, rail's, butters, and hitting stuff "unconventionally" the ep's were better, they are lighter, poppier, more lively, and amazing for butters, you can get into a nose butter and hold it for like 100 feet, its awesome doing crazy butter combo's on knuckles, boxes, etc....i was hitting 50 footers with both no problem, you can land pretty far backseat before they actually wash out, and they both carve like crazy off lips.  it seems like the hellbent is more of a park ski than the ep, like it was more designed for it, the dimensions are perfect, and the flex is perfect for all around. the feeling of the hellbents on jumps is amazing.
soooo........if you want the feeling of a more traditional ski with the ease of rocker in pow go with the hellbents.
if you want a very different experience and a very surfey, super amazingly playful feel, go with ep's.
if you want to hit mostly jumps in the park go with hellbents, if you want to do more rails, butters, and all mountain jibbing go with ep's.  (not that ep's are bad at jumps, they are awesome actually, just not as cool feeling as hellbents)
to me i think ep's are the better ski, it totally changes how you can ski powder and it makes you look at the mountain and park in a whole different way.
wow that was long 
