Let’s initially say you want the maximum safetly available today. Actually, I think it’s pretty easy to establish some respective general principles if you ignore a few of the less credible (imo) manufacturer claims:
1) No system covers all threats.
2) But a filter plus a solution for viruses covers everything except (imo) chemical pollutants. Meaning: tape worm eggs down to bacteria get caught in the filter, and viruses get killed separately by a Steripen or tabs/drops.
3) The Steripen is significantly faster than tabs for viruses, but also far heavier and more likely to fail.
4) Avoid the source entirely if you suspect chemical pollutants.
So in a group, it’s a bit of a no-brainer – take a filter, a Steripen and some backup tabs. Solo? Take a filter and tabs… then add a Steripen if you want to speed things up (and there are certainly times when you will appreciate that factor).
(The game changer with regard to chemical pollutants would be Geoff’s Seychelle system – if we can really trust the claims. On the one hand, it’s usually fairly easy to figure out which sources could be polluted – and they often coincide with the vicinity of human settlement (so you could probably acquire tap water fairly easily). On the other hand, that rule isn’t foolproof – so I, for one, would swap my Sawyer for Geoff’s filter if the claims could be further substantiated.)
Next step? Start to eliminate options to lighten your pack. You need to base this on the threats that are actually likely during your trip. Is it worth it? That’s up to you. But here are some considerations:
1) Steripens need clearish water, as do tabs. Neither cater for tapeworm eggs (yes, I know… my personal ‘favourite’).
2) Tabs are much slower than Steripens, especially if the water isn’t filtered – up to four hours to catch all protozoa (as opposed to 30 mins if they only need to deal with viruses).
3) Viruses may or may not be your smallest worry, depending on your region.
4) As a backup system, tabs weight almost nothing.
5) Filters can clog, and Steripens can fail.