+1 for the Hiker - I have a Hiker Pro and a gravity filter from ULA that use the Katadyn filter element. Some trips I take the pump, others I use the gravity filter. I find that the gravity filter is just cool - I filter once in the evening, and while the bladders are filling I set up my hammock, unpack the bear can to arrange dinner, set up the stove, collect fire wood, whatever needs doing. While I'm doing all that the filter provides enough water for dinner, breakfast and the next day on the trail. When I want a shower, I remove the cartridge and attach the shower nozzle, hoist the filter bag higher, and use the valve to control flow. Granted I don't hike in the desert and water sources are everywhere in the Sierra. Late in the season, when sources slow to a trickle and I'd have to bail with a teaspoon, the pump goes with me instead.

The decision for me was one of practicality and minimizing the chance of error on my part. MSR filters and Katadyn filters rate about the same in filtration capability. You have to keep both from freezing. The difference for me was the "duh" factor - I don't want to have to dismantle and scrub and reassemble an MSR on the trail. The Katadyn has a screen around the filter cartridge to keep it from gumming up prematurely. Most of my sources are clear enough that I haven't even had to clean that. The Hiker Pro is simple to use, the gravity filter is simpler, both have fewer parts than an MSR, and the filter cartridge has a carbon core. The tradeoff is replacing a 35-40 dollar filter cartridge every so often.