I prefer oiled foam over paper, they are more efficient at filtering, and can be cleaned and reused a few times. I have a UNI filter (in the stock unmodified airbox) on my dual sport bike, and at the end of a day riding in the AZ desert it will be caked with so much dirt that chunks of oil soaked dirt have fallen off and are laying in the bottom of the airbox. A paper filter would have to be replaced, and they are not cheap. But on the Rebel, which is not ridden off road, the paper filter seems to work just fine. As for K&N, I wouldn't go anywhere near one of those things. They will stop small gravel, but that's about it. They are advertised as a high flow filter, and that means dirt as well as air. But you want to keep the same airflow as well. More airflow will lean out the air/fuel mixture, and the carb will have to be rejetted to compensate for it. Unless you are into that kind of thing, it's better to just leave it alone.