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side wall skis are more durable, no doubt. cap is cheaper (and for a good reason). lemme break down some ski construction for you:
cap construction is just a wood core with a base glued to the bottom and a plastic and fiberglass *cap* pressed on top under high pressure. the result is a light, cheap, evenly flexing ski.
sandwich/sidewall skis are essentially the same basic construction (wood core with same kind of base) but ABS or similar hard plastic sidewalls are added over the edge, and the press usually presses in four directions, resulting in a tighter fit to all the parts. this adds TONS of durability to the edges.
go sidewall unless you can afford to replace your skis after a season (or even during a season)
note: salomon and head offer skis that *look* like caps, but are again a slightly different ski construction, and i havent had any bad luck with those.
hope that helps man.