I use foam. I think it stops more dirt. It all comes down to how often you clean it. If you neglect it, either one will pass dirt. Clean it often and neither one will pass dirt.
Keep in mind most people like to run an outerwear pre filter wrap in addition to the k&n....why??? You know why...... and at that rate, whos now getting more air, the k&n or the foam?
Another thing that should be addressed is how the person oils it. I have a buddy who just cant seem to oil his filter right. Its usually overkill or not enough in some spots. The oil is what stops the dirt. Too much kills air flow and clogs up the intake track.
I also think a k&n breaks down more than a foam filter in time. Foam keeps its shape/pores. The K&N starts to seperate the fibers down the road. K&N's are also ALOT more money to purchase.
This is my opinion. This topic is like asking people what their favorite color is, you will get several differnt answers and everyone feels their doing the right thing or they wouldnt be using it