You definately have a great point about the weight of the 159... once stuck, its over.
I would argue that the 159" does provide better floatation and given the right amount of horsepower should outclimb and 136" with the same weight and power. Personally, I think a 144" Vertical Edge 800 would be my choice for mountain riding just because, the 159 is indeed HUGE, but if you hit that super heavy powder, I think the 159 will always outgo the 136 given the same sled weight and horsepower.
Where do you ride? Any pics of your 450 pound 136" must be a custom sled.