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^thats the Z series of bindings your thinking of. The STH heel was introduced to help prevent the cracking which was and still is an issue Z series bindings.
The STH are pretty much the best bindings you will find at the moment. People will argue that FKS are better, and as a pure binding they may be, but when you add in how hard FKS are to fix, it makes the STH a clear winner in my opion.
I think your right not to go down the second hand route. Yes you could find a deal, but you will have no warrenty should somthing go wrong, and you also have no way of truley knowing how hard the bindings have been used. They may comestically look ok, but the springs could be done.