The main difference between a CAI and Short Ram is where they pull the air from. The Short Ram pulls the air from the engine bay and the CAI pulls the air from below the engine bay. The air in the engine is usually quite hot, so many people believe a CAI is better for horsepower gains because the colder air compresses more. A lot of people also agree CAIs will be louder. I personally have the K&N which calls itself a Short Ram even though it isn't really. It pulls the air through the top of the grille. I'm not sure exactly how low the AEM/Nismo and Injens sit, but I do believe they are lower than the K&N. If you decide to go with one of these two, the AEM bypass valve is highly recommended. I think most Z owners have opted for the JWT which is a true Short Ram. It is by far the cheapest of the intakes.