Hmmm on the gravity filter. Been there done that - They are slow and fiddlesome. You need a place to set it up and you have to watch the bottle so they don't fill and fall over and did I mention slow? If you need water now and can't wait for half an hour, carry a pump. With a pump the pressure you apply can overcome some pore clogging, but with just gravity the amount of pressure may not be enough if the filter starts to clog. Water, reliability, long term storage requirements, ruggedness and ability to be back flushed and cleaned have lead me to carry a PUR backpacker for over ten years now. It is heavy - around 11 oz as I recall - BUT as I said - reliable rugged and fast. When you're squatted by a lake in a swarm of mosquitoes you don't need slow...
These are my own opinions based on wisdom earned through many wrong decisions. Your mileage may vary.