I currently ride a 158.5 Rippey and I'm 5' 9" and about 170-175lbs. I can't decide between a 155 or a 158.5 if I go with this board. NS suggested the 158.
What size boot? You and I are the same height and weight, I have a size 9 boot. I own three NS boards (SL, Heritage, and Ripsaw). My first was the SL 155. I rode the crap out of that board in east coast stuff. Going out west I picked up a 160 Heritage. I mainly do all-mountain stuff and am pretty directional, and I did well with it, though I felt it was at my top end for size.
For the Ripsaw I chose a 156 over the 159. At first it felt on the small end, b/c I was coming off of the Heritage, but by day two it felt perfect. I mess about more on it than I would the Heritage.
Back to you...if you're a more all-mountain/groomer type of rider, go with the 158. You're already used to the size, and the Cobra is pretty agile. If you tend to mess around more, do quicker turns, trees, etc... then the 155 can work too. Honestly either one will be great for you, but the 158 will probably feel best right away.
Oh, and to the OP, 158 for you as well I'd suggest.