I have used ULA's gravity filter the Amigo Pro and very much liked it - I misplaced mine but will get another when I'm backpacking again in spring. The cost is about five dollars more than what one would pay for a replacement Katadyn HIker Pro cartridge. That filter impressed many in my hiking group. One of them picked up the Katadyn gravity filter, but that weighs a lot more than the eight ounces of the Amigo.

I also have a Hiker Pro that comes in handy late in the year when water sources can be very shallow and not suited to scooping water.

Using chemicals in addition to filtering is what the MSR Sweetwater system does, as this addresses viruses too. I am usually in the Sierras where the issue is mainly bacteria and cysts, which the filters all handle. I do carry Micropur tablets in my emergency kit so if a water source I am forced to use is suspect, I would likely use the tablets in addition to the filter.