If it means anything to you, I'm 5'10" ~180+ lbs and I have the 155 CTX. It's definitely not too soft by any means and decently stable from what I can tell. I actually expected it to be much softer than it is. I feel if you ride mostly groomed runs or steeper/quicker runs it might pay to get the 158, but it really doesn't bother me that much at all. I like the fact that with the smaller board, it can go on any area around the mountain with no concern. In all honesty, I think the difference will be small enough so that you will be happy with your decision regardless. My advice would be:
-Ride more park / trees: 155
-Ride fast / groomed / steep: 158
I had the same decision to make last year, but then the 158 went out of stock. lol made my decision a whole lot easier.