I opted for the Lingenfelter. Reasons:
1.) A heat shield comes with the kit.
2.) Plus the following Quote from Lingenfelter:
The Lingenfelter high performance Pontiac GTO air intake provides an easy to bolt-on 12 HP gain at the rear wheels.
The easy to install, bolt on air intake system is designed to fit the 2005 GTO. This low restriction air induction system has shown in back to back testing 12 Hp, and 9 lbs ft. of torque over the stock GTO air intake system! The system is designed to provide lower temperature air from outside the engine compartment in order to maintain optimal performance.
3.) Lingenfelter Specializes in this type of engine.
The K&N estimates a 13.5HP gain but at the engine and not at the rear wheels. K&N also uses a new intake tube, they claim the factory airpath to be too restrictive, however this is based on all vehicles, and is applied to the GTO.
I am sure they are both good, I just prefered going with Lingenfelter since they did real world testing and they DO specialize in this engine.
Everyone here will get different results and have different opinions. Thats just my 2-cents. :cool