I'm almost exactly the same size as you (I'm 5'10" 140lbs), and I have 08/09 179 hellbents, which run 5cm longer than advertised, and I ski crystal as well. I would reccommend going for the 189s, because the new k2s run true to size, and I sometimes feel like my 'bents are a bit small. Although if you really want to bring out the tree skiing performance, by all means go for the 179. However, I would reccommend looking at some other skis besides the hellbent. Their softness makes them super fun in pure powder, but as soon as it gets tracked out, you can expect to take a bit of a beating. I would look into skis like the on3p caylor, moment night train, atomic bent chetler, klint krypto, salomon rocker 2, etc if you want a bidirectional pow ski. These will all be a bit stiffer so that they can cut through the chop much better than the hellbent, while still being super fun in powder. The new k2 pettitor looks super fun too, if you can hold out until next season. If you'd be fine with a more directional ski, I'd take a look at the on3p billy goat/wrenegade, moment bibby/jaguar shark, atomic atlas (or hold out for the 12/13 automatic), or some of k2's adventure series like the darkside/sidestash, etc. There's really no need for a twin tip if you're only going to be skiing forward, and they can be easier to deal with in the backcountry with skins and such. But yeah I would definitely look into a slightly stiffer (at least stiffer than the hellbent) pow ski for skiing in the northwest. You also don't need to go quite as wide as the new hellbent. Anything around 120 in the waist with a decent rocker is going to float just fine, and will be a lot easier to manage in choppy stuff and on hardpack getting back to the lift. Cheers.
sparknotes: if you're going hellbents, size up, but look for something slightly narrower and stiffer. Also check out flat tails, although if you're considering the hellbent, your probably going to want to ski switch...
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