Go with the 170 cm length.
Here's a free "lesson":
A ski that is decambered into a curve and tipped onto it's left edges, say looking like this from above ")", will cut a turn to the left. A ski that is decambered into a curve and tipped onto it's right edges, say looking like this from above "(" will cut a turn to the right. It's always been that way, but it seems that a large percentage of the skiing public including instructors (97%?
) didn't realize this and thought it necessary to twist the skis into a new direction.
The snow-plow turn is just a stepping stone to get you used to balancing with forces acting on a ski's edge. The stem-Christie, is just a stepping stone to get you into a position with curved edged skis turning you at the end of a turn. Ski fast enough and you should realize that all you have to do is bend the tips of the skis and tip them, and drop the stemming! Spend too much time at the stem-Christie phase and you may never un-learn that stem. Of course, some people don't have balance skills and need to ski in a gliding wedge for a while, but that's another kettle of fish with it's own problems.
The only difference between the new more shapely skis and the old straight skis is that with the new skis having greater side cut, tilting the ski on a hard snow surface by itself is enough to decamber the ski by an amount that will initiate a strong turn. With less shapely skis (aka "straight" though they were not really straight), you needed a few extra tricks to get the curve into the ski at lower speeds. The only trick that was really needed was to bend the tips with forward weight bias in conjunction with tipping, but twisting an untilted ski seemed easier for some. Hence, the twisting the ski into a new direction referred to above came into being, and spawned generations of skiers stuck at a plateau who would never learn to carve a proper turn
All you need to do is tip the skis. If you're not comfortable with that, go ahead and use a stem-christie until you get the feel of riding tipped skis, but if it takes more than a couple of hours, you need some lessons.