From what I've read the UNI does a real good job and seems like it's preferred over the K&N.
But like Steelslinger said, it does depend a lot on the area you ride in. My area has very fine silty dust which means no getting away from an oil impregnated filter.
For fine dust, I think the UNI. For better air flow, K&N.
If you keep it clean, it really doesn't make a lot of difference. When you pull the filter to clean it, look to see if there's any dust or silt in the box. It should be clean if the filter is doing it's job!