Im the same size as you and had the same problem a few months ago, I went with the 161, personally i think its the perfect all round size for all mountain riding. I found the the board actually roade a little shorter than its length which was especially evident in the pow trees of japan.
At your size I would hesitate at going for 158 unless you do want to spend a bit of time in the park vs the all mountain terrain. Im definitely glad i went with the longer option. Plus as mentioned above sooner or later youre going to make it up to Japan, and there you will prefer the longer length for the pow floatation.
Looks like there will be quite a few SL-Rs downunder this year, I only spotted a legacy on my trip to Hakuba this year, so theyre still a rarity on this side of the world.
Youll love the board so jump on a deal that you see as most are already gone, luckily i picked mine up before xmas from backcountry who shipped to Aus but i dont think they have any stock left.