Pete,
I've used the Hiker and Guide filters. The filter elements seem to work very effectively under the pressure of the pump. The Guide filter in particular, can fill a Nalgene bottle in a very short period of time. I wonder how well the gravity feed system, using the Hiker's filter element, will work when the filter has been used for a while. As it starts to fill with "stuff", will the gravity feed become painfully slow? I've never had one of the pump style actually clog to the point where they couldn't work, but, I could imagine them being filled with enough gunk that gravity wouldn't pull fluid through them.
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- Ron