Hi Birdlegs--Welcome to EpicSki!
You could ski the S12 in either length. The 176 would have more flotation off-piste, and theoretically more stability on the groomed, but the 168 has a tighter sidecut radius, and at all but the highest speeds would feel like a higher performance ski. If you're concerned whether the 168 would be "enough" ski for you, again, at all but very high speeds, or deep, soft snow, it would.
The standard model of the S12 has a beefier, more solid feel than the Fusion integrated binding model. Try them both--it's a matter of personal preference.