If there's not much difference between the prices and specs then he should go for a DD as it's what everyone rates at the moment, however, like Steve said if there is a big difference then I would concentrate on the dyno room.

On another note, do you think DD's popular over here solely because of the product or because of the product and Mike Gurney? It'd be nice to hear peoples opinions as DD's aren't popular in America compared to Mustangs/Dynapacks/Dynojets