GRS would be my choice. Personaly I would'nt touch Airtec.
I know of an owner of a S2 that tried to improve upon the efficiency of his GRS Evo3 intercooler.
Switched to the Airtec and lost 80bhp @ the wheels YES at the wheels, SHOCKING!
He then had Graham make him a Single pass GRS but he sold his car before fitting it.

**edited** just to add the car was a 2.1 zvh running a T38 and a shed load of boost. Between fitting the airtec and finding out just how bad the airtec actualy was, he sold the GRS, so couldnt go back to what had worked well in the first place.