the 155 is an all-around burlier binding. For example, with the pressure release rate of a FKS 120, I would have to have my din on 17 to keep me in safely. But with the pressure release rate of a FKS 155, my din is around 13 to keep me in safely. On a FKS 185, my din is around 8 to keep me in safely.
the higher the din, the burlier the binding. The 155 and 185 have a different design from the 120. The 120 is pretty much a plastic version of the 155, and with a lower din.
I have never used 120s, I always rode p12 Jibs (which I cant use anymore), Fischer 14-din bindings, and FKS 155s. This year I snagged a 2nd pair of 155s so Im stoked to use those in the park. the Fischer 14s kept popping out like a mofo, even at a 13 din.
so to answer your question, you are going to have to honestly evaluate your size and skill level as a skier. If you shred like no other and are around 5'8", 145 lbs, then get a 155. But if you're more towards the easier side of skiing, then I suggest the 120.
whatever choice you make, keep in mind: they're your knees.
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