I think my PUR is as fast as a steripen, AND there is ONE HUGE advantage to a filter - it filters. Yah - all the junk in the water is still there with anything but a filter, so if your water isn't pure enough to drink out of the stream, you might want to filter it anyway. If you don't have enough room in your pack for a filter, you should consider getting a larger pack anyway.
Jim
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These are my own opinions based on wisdom earned through many wrong decisions. Your mileage may vary.