I have the 15” stick topped with an angled Monarch platform. I would say it could depend on if your top step will be a DoubleStep or a platform.
At 15”, the stick is extremely packable and handles nicely when setting it further up the tree. However, for me the platform does knock my shins when I’m trying to stand on the bottom step and advance my tether during my climb before stepping up onto the platform. I end up having to do a bow-legged straddle with my toes pointed at 45° angles and my calves pinching on either side of the platform and the tree. It’s not the most comfortable, especially when wet or in the dark, but I prefer to advance my tether as I climb and minimize slack. At those times, I wish I had an 18” stick because it’d be a more natural position on my leg/knee to help grip the outside of the platform. But, if I’m ever climbing directly from bottom DoubleStep to the platform, it’s not a problem.
I’m not saying 15” isn’t good—I’ve found a way to make it work for my one-sticking style. Yours might be slightly different. Moreover, depending when you advance your tether and if you have a step or platform up top could nullify that problem.