Visiting family in South Asia, or travelling in Mexico or Southeast Asia? The difference between a filter bottle and a purifier comes down to one word: viruses. Where tap water isn't safe, use a purifier that removes them.
Where the water isn't safe: a purifier
The GRAYL GeoPress Purifier removes viruses, bacteria and protozoan cysts, according to GRAYL. Fill it from a tap, press for about eight seconds, and drink: 710 ml at a time, with a cartridge rated for about 250 litres. That's the one we'd take to India, Pakistan, Bangladesh or Mexico.
Where it's mostly fine: a filter
A basic filter like the LifeStraw Go removes bacteria and parasites, but not viruses like hepatitis A or norovirus. That makes it great for Canada, the US, Europe and hiking, where viruses in tap water are rare, and not enough on its own where they aren't.
Backups worth carrying
A strip of chlorine dioxide purification tablets weighs nothing and treats a full bottle if your cartridge runs out; just follow the waiting time on the pack. For places with safe tap water, a collapsible bottle folds flat for the trip home.
Two things a bottle won't do
Carry any bottle empty through security and fill it after. And a bottle doesn't replace your travel vaccines; get your hepatitis A and typhoid shots before a trip where they're recommended. When in doubt, sealed bottled water is still the safest bet.