this is copied from a post I made the last time this discussion came up
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 lighter, but the AR7s are still light as far as skis go
ar7s are brand new this year so not many people who reply to this thread have actually skied them unless they are close with Armada
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. I will buy S4s as my park ski till I die though. Within a tenth of a pound of being the lightest mid 80mm skis and the most bomber construction you can buy.
you will get edge cracks no matter what ski you buy, it will eventually come to you one way or another. And I have the ar7s, and love em so far. I expessially like the construction, i think they will hold up very well