I live in Pennsylvania, so I'm mostly riding man-made snow with the occasional bit of powder...groomers and ice is pretty much everything I have to deal with.
Fair enough, in that case I'd say if you really want to lay a good carve, longer is better, but if you're wanting to play around on it, go for the shorter one. Again, I'm 5'7" 175 lb and ride a 160 and it certainly doesn't feel too long to me, but I'm way more into freeriding.
The trusty snowboard calc
puts you in the 160-162 range based on your weight, ability, etc. As mentioned in other threads the calculators are just a guide, but I've found them to be pretty damn accurate so far.
My vote goes for the 158 but that's just me...