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What skis are we talking about? What is the 168, and what is the 179? That matters because length isn't the only factor that one has to take into account when figuring out an appropriate length. A softer ski is much more easy to handle in a longer length than say, a longer ski.
Also, what's your weight? If anything, weight matters more than height in determining proper ski length because it basically determines how well you'll be able to power the ski and take it where you want to go.
A longer ski is harder to control - that's the biggest thing. Tell us what skis they are and how much you weigh and then we can give you a straight answer. For all we know you could be ordering two-by-fours and weigh 80 lbs...