Ya know, provided that you tune the pump right, and since the GT15, the IHI and the KO3 are comparable in size and flow rate, it really comes to what you have access to and your preference.
This is only anecdotal, so take it with a grain of salt.....the IHI has the best response out of the three.
I know you'll engineer the turbo system properly, and that is going to determine the response time more than anything.
EDIT: Go with the one that's closest to what VW did ( GT15 or VNT GT15 ),
saves a lot of trouble.