Here's three overview articles worth reading:
Filters are like handguns. It helps to understand the nuances. They all work, some better than others, some very much better than others. And some deliver specific advantage for certain scenarios better than others (like international travel to the third world... or grunt use). Different mechanisms, sizes, weights, prices, and capabilities.
The first practical backpacker filters came to the market from companies like Swiss firm Katadyn in 1957. Followed by later offerings from General Ecology, Inc. (First Need filters) and MSR (Sweetwater Guardian filter) way back in the 1970's. Those three plus latecomer Sawyer (2000's) dominate today's world market for man-packable water filters. I trust the products of all three of those older firms the way I'd trust CZ, S&W, & HK semi-autos. Sawyer is the new kid on the block and they've essentially marketed the Glock family of water filters. I trust their stuff too.
When it comes to high end (cost & performance) hand pump filters for individual or small party (fire team) use, two come to mind. Neither are inexpensive...
1. At the high end of performance (and price), the MSR Guardian is arguably the current king of the hill. It's MSR's flagship filter. Flow rate and level of filtration (including elimination of tiny viruses) is fantastic. Self cleaning back flow feature with every pump stroke. So no need to back flush. 17.3 ounces. 2.5 liters per minute flow rate. Medical-grade fibers in the Guardian block viruses—something most other hollow-fiber filters cannot do. 0.02 micron filter physically removes viruses, bacteria, protozoa, and particulates. Rugged construction is designed to withstand freezing, withstands heat up to 160°F, resists up to 300 lbs crush force, and survives drops up to 6 ft. Advertised 10,000 liter cartridge filter (when filtering already clear water). Closer to 600 liters per cartridge if filtering crappy water.
Originally built for U.S. military squad use, the Guardian meets US military NSF Protocol P248, for removing viruses, bacteria and protozoa in water abundant with silt and particulates.
But...mostly Plastic Fantastic construction. So maybe... perhaps... not so GI-proof. Time will tell. Recent introduction (less than 2 years ago) with a few internet noted seal leakage problems. In other words, a couple of factory lemons slipped out the door. The problem was merely episodic and is possibly solved by now. ~$350.
2. The classic (since 1957) Katadyn Pocket is the recently displaced former high dollar champ, but still the go-to model for those that need bombproof durability (steel/aluminum construction). Like a USGI 1911A1, it's Old Reliable and pretty GI-proof. 19 ounces. 1 liter per minute output rate. Field cleanable silver impregnated (kills critters) ceramic filter good for a theoretical 50,000 liters (with occasional scrubbing of ceramic filter). 0.2 micron filter eliminates bacteria, protozoa, cysts, algae, spores and sediments (but not viruses). You still need to boil or chemically/UV treat if facing viral threats like Cholera or Ebola. 20-year guarantee.
The Katadyn Pocket was adopted by SOCOM units (including Army SF) back in the late 1980s. As far as I know, it is still in service with those units. Katadyn has been selling this same basic filter (with a few updates) 60 years. Because it works and doesn't break. Also ~$350.
The Guardian mostly beats the Katadyn for military use because it outperforms it on technical points. A much finer filter medium screens out viruses the Katadyn does not. The MSR Guardian offers an ingenious mechanical self cleaning feature. It also has a much higher flow rate (the advantage of which I discussed up thread).
But to recap that point... the MSR could fill-up & filter a 5-man fire team's canteens and bladders (call it four quarts per man) in about 10+ minutes. The Katadyn would take at least 20+ minutes to accomplish the same. But a hell of a lot more forearm workout would be required. Which means pumping rate is going to slow down the longer your guys are at it. Meaning they'll have to rotate pumping duties. It might actually take longer than 20+ minutes. Half hour or more. More physical movement and time expended at the water source or patrol base halt. BTDT using that particular model of Katadyn.
Weight & price of both filters about the same. But...
If you've got the kind of sleepy-eyed killers who could manage to break and impregnate a bowling ball... or you need something that doesn't require more frequent resupply of shorter lived filter cartridges... or need something that is going to last forever after spending big bucks for it...the Katadyn Pocket is your pony.
Say I were equipping an 8-12 man deployable SF ODA to go out the door tomorrow to Afghanistan or desert Africa. For several months or a year. I'd equip each man with chlorine dioxide tablets and a Sawyer Squeeze personal filter. I'd further equip that team with 2-4 MSR Guardians (1 per split ground element or 1 per vehicle). Now for the follow-on logistics:
I'd consider each MSR Guardian cartridge adequate for providing just one person about 150 man days worth of water cabbaged from dubious sources (at 4 quarts gathered per day). Or about five weeks use from one filter cartridge supporting four men. That's more $180 filter replacement cartridges that have to be carried or resupplied for an 8-12 man team across the long term. Cartridges that aren't heavy, but like radio batteries, you'd best not run out of them. Or lose them. Because, worst case, that 600 liter per MSR filter cartridge number is probably the correct planning factor when drinking out of muddy silty streams in the Hindu Kush or animal/human polluted water holes out in the deep Trans-Sahel. And you ain't finding those replacement cartridges down at the local indigenous shop or outdoor market.
The MSR advertised 10,000 liter figure means treating already relatively clear water at some inhabited location (like a rented safe house, third world hotel, or village well). You might make a filter work for several thousand of those liters before needing to replace a cartridge. Or you might not. For continuous field use, I'd bet on not.
Another thing to consider...
We just might find our infantry on a future battlefield where pallets of reliably delivered bottled water and well supported FOBs don't exist. For instance, one where US theater resupply is interdicted by PLA naval and air assets. Think Guadalcanal, Chosin Reservoir, or Corregidor instead of LSA Anaconda, Camp Lemonnier, or Bagram AFB.
If I were inserting that same SF team into an indefinite duration guerrilla warfare campaign with problematic resupply (like WWII Japanese occupied Philippines) ... I'd take Katadyn Pocket filters instead.
For the shorter haul, the MSR is the better performing filter. Across the long haul, the Katadyn is more practically durable military choice. Might just be Ford vs. Chevy concerns, but... nuances.
Just my opinion. (And I've ordered an MSR Guardian for personal hiking use.)