Mohair is supposed to give better glide, i.e. for longer tours with flats while nylon is supposed to give better climbing traction. My wife has a set of the Black Diamond GlideLite STS Mohair/Nylon blend skins that we got for a great deal on tramdock. She has no problem with traction on the same slopes I'm climbing with my nylon G3 Alpinist skins. You'll probably be fine either way, but nylon seems to be a lot more common.
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