I've always been a foam filter guy for everything I build. I have a pipercross in my XB and have used HKS's in most of my auto builds. They are the easiest to clean and the highest flowing while still capturing contaminants. It's all in the cell structure.
For those they want some serious science http://duramax-diesel.com/spicer/index.htm. K&N lets a lot of contaminants through but does flow the most. The important thing to note is how fast it clogs up though. It will flow the most but for a relatively short period of time. Basically K&N will get you the most power out paper filters but will also let the most crap into your engine and require you to clean and oil it all the time.
