I have tried both a HKS and a K&N, I much prefer the K&N.
The "recommended" K&N filter is relatively small, the one I bought is probably the biggest you can fit in the FTO, from memory the guy said it was designed for the Holden Calais. I went for a bigger one, as it has a greater surface area, which means it will breath easier and filter better.
An air filter is a fine balance between restricting dirt into your engine, while allowing air through. Typicaly the more it filters the harder it is for the air to get through, so you need to way it up and make a choice.
Here's a couple of links you may be interested. Unfortunately they don't do a dirt test on the ITG pod.
http://www.ftooc.org/contents/pages/project/filter.htm
http://www.mkiv.com/techarticles/filter ... index.html
Because of its bigger size, the K&N filter I've got will work better than they one they tested. I like my engine clean, I like the value for money in having a cleanable / re-usable filter, and I like the performance gains I got, so I'll be sticking with my K&N.