Go with the longer board. You're not in the park. You'll have more stability and better flotation in powder with a long board. The benefits of the longer board will greatly outweigh the slightly less maneuverability.
I bought a 158 SL at the end of the season last year after demoing a bunch of boards. I demoed the 158 Legacy, but it's what they had to demo. You'll notice a huge difference in performance from the noodly Evo to the SL. I did a similar transition.
I weigh 160, ride a 158, but never see any powder in the MidWest.