Typically, ducting has a ribbed design all the way along it, to allow it to flex and bend etc. This makes it easy to route, but also disrupts the way the air flows. The air will flow a lot more freely with alloy or stainless piping due to the completely smooth design.
The performance difference between the two would be fairly minimal if using the ducting as a main induction pipe as opposed to using alloy or stainless, but the least disturbance to the flow path the better, so I reckon it's just better to spend a bit extra on getting the metal piping. Plus, as a bonus, metal piping looks awesome in the engine bay

. Feel free to check out my CarDomain page to see the type of ducting I used (was reinforced with metal wire) and also to see the alloy piping I upgraded to.