I think you could/should go for the 158. I started this year and I'm 135 pounds and learned on a 155. Given, you could take the 155 and it would help you pick up snowboarding faster, but since you want to do a lot of all mountain riding, the 158 would suit you better in the future.
EDIT: Just wanted to clarify that I did not learn on a 155 NS SL. I learned on a 155 Rossignol noodle so it would be a bit more forgiving than an SL presumably.