You could really go either way. I'm 6' 185 lbs. and am on the 178, and it's perfect for me. I've also skied the 188 and it was a blast, but just wanted to go fast and make big turns for me at 185 lbs.
If you're really not concerned about stability, then the 178 will be more playful and relaxing. The 188 may be much better for you at 220 lbs. on hard snow though. It's not a ski that will beat you up, even the 188 cm, so you might want the 188. You also will be able to get the 188 for less money than the 178.
Sorry for adding to your confusion You'll be happy with either length in the end. Great skis!
I am impressed by what I am reading on these skis, AND love the looks of them. Plus prefer an Austrian ski if possible. I want to demo some Cimson TIs, RTM 84s and Rossi E88s. Yet wear I ski in NY, the do not have the Kastles to demo. If I would purchase them online, what length would I need in your opinion? I am 6' tall and avg 220 - 230 lbs in winter. And am an advanced skier. I am not so concerned about stability at speed, because I am a fun, relaxed skier, maybe hitting some speed on the blues. I would rather go shorter for easier short turns rather than long GS type turns. And want something easy to handle when I take lessons for steeper terraine on wide, easy black runs.