Neither of these two products will give you colder air. As most Rover owners know, the underbonnet temperatures are really hot, due largely to the Aluminum engine. (Aluminum being a better conductor of heat)
Taking the engine air from Outside the engine compartment is what you're after. The colder the air, the more dense it is, and the greater the O2 entering the engine. Any Snorkle (Raised Air Intake) will get you that, however some create a velocity increase in the airflow (Mantec's Snorkle top acts much like a velocity stack) All RAI manufacturers will claim free flow ability, but better than free flow is an increase in flow.