I'm surprised the 160 won't float you in powder. I've ridden my 165 wide
in every different powder condition and flotation has *never* been an
issue. OK my board's bigger but then I'm 6'5" and 185 lbs! Could it be a
speed/technique thing - you say you don't get much powder at your hill:

The Incline 160 did not float me in powder either, but the Wide 161 floats
me very well. The two boards have the same sidecut and stiffness... only
different in width, nose length, tail length. That extra width seems to
make all the difference for flotation!

I mostly freeride, and there's not that much powder at my local hill,
but plenty of trees.

So with the manoeverability problem in trees, are you riding ice/crud in
there or is it powder? If it's the former then shorter with more flex
should help. If it's the latter, then I agree with the Fish suggestion -
but I do wonder if it's just a question of speed/confidence on your
current board.

Either way, I think you'll find a wide 156 will be too similar to your
Incline 160. It will still be relatively stiff, making it not maneuverable
enough for you in the trees. If you want to stay with Donek - perhaps you
might look at a Phoenix? (And tell us all about it if you do, many of us
are curious!)

BTW, I find my Wide 161 "just right" in trees, but I am heavier (195
pounds), and have about 400 riding days under my belt, and I tend to like
stiff boards... so I am not surprised at all that you find it a lot to
handle in trees at your weight, and doubly so if you're not as experienced.

Mike T