Traditionally, sandwich was much stiffer in torsion than cap (talking about the eraly Salomon caps). One of the reasons, and probably the main reason, was that many of these ski used lighter foam rather than wood as their main ingredient in order to cut production cost and make the skis cheaper to produce and ship (the Axiendo series, for example, were very light, while still being lively).
The older Dynastar consumer race skis, or the Omecarve, featured a best of both world construction: cap in front and behind the boot, sandwich beneath the boot, in order to have a stiff ski underfoot that would be softer (and easier to initiate/skid) in the tip and tails.
Nowadays, "real" cap construction, the monocoque deal where the skis has an exoskeleton instead of a proper core followed by layers of wood/metal/foam to change its characteristics, is practically gone and we have skis like Atomic and Salomon race stock that can be very stiff, even stiffer than other sandwich race skis. The Atomic venture into cap dowhill race skis last season proved that you could make a comparable, or even a better, race ski, even in the speed events, wich still is 100% sandwich.
PS: There's more to the discussion than just cap and sandwich, there's also torsion box, oversized sidewalls, different materials (Head's liquimetal for example) and technologies (Frequency tuning, Chip, dampening system, Beta lobes, etc.). The difference is much more tradition and personal preference than performance nowadays.