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sidecuts are basically identical
the two most durable rail skis you can buy
full disclosure- I've never skied ar7s. I own several pairs of S4s and I've spent a bunch of time on ar6s.
ar7s are a bit stiffer
s4s are maybe a touch lighter, but they are both crazy light as far as skis go
ar7s are brand new this year
S4s haven't changed since 2007 to my knowledge (which isn't really a bad thing, AR6s were around for a while and they were sooo sick)
you can't go wrong with either