I have had both fitted to the Trophy, the Induction kit will just lose power all day every day. I have a K&N Panel filter fitted minus the A/Valve and when it was last RR the BHP was up by over 4bhp compared to the IK.
IK are made for the Boy Racers of the world. The only cars that benefit from Open Air Kits are those running turbos, where the engine benefits from the larger volume of air flowing throught the Turbo. As you said they sound good and look good, but if its performance your want stick with what Renault designed, unless you have one of the ST150 Fiesta then chuck the Ford set up and put a IK on.
The Renault air box is actually one of the best less restrictive sytems out there, its quite a direct flow into the throttle Body, as oppsed to the ST150 which has a pipe that runs from the top of the engine down to the TB.
Also look at changing the elbow for a less restrictive Samco or hill power bend, wont make a huge difference but every little helps and looks great under the hood.