The UPR fitting and the valve cover fitting is 5/8", so the 1/2" ID breathers will be a very tight fit. AirAid only has a 1/2", while K&N has a 9/16" (but the filter itself is small) and a 5/8" along with a 1/2". I like the AirAid better because the media seems to filter out all the dirt. I tried a Green Filter breather and there was a ton of dirt inside the valve cover port, inside the filter, etc... because Green Filter is garbage for filtration. I also used the AirAid pre-filter wrap with the Green Filter.
I'm running the AirAid with the AirAid pre-filter wrap and have a K&N 5/8" breather as backup. The K&N breathers actually use the AEM dry flow material now. When bpd1151 wrote his article, K&N was still using the oiled cotton gauze, but they've switched to a woven synthetic now that is dry. AirAid is a combination of synthetic with oiled cotton gauze.