Being that you're 176 lbs., I would go with the 159 if you plan on hitting the park and riding all-mountain freestyle. I haven't ridden the Mayhem wide, but I have ridden the Burton Bullet, a wide all mountain board very similar to the Mayhem, so I can only give my opinion on the Bullet. I like it, although it is a little heavy. It does very well in the powder, decent flex, and mediocre pop.
With only six weeks experience, I think you'll like it.
I am curious though, what size boot are you wearing?
I'd imagine a size 12 or so, being that you're 6'2"