Purifiers kill pathogens. Filters remove them (OK most of them - not viruses). Purifiers like the Steripen (
http://www.hydro-photon.com/) sure look easy but I don't think they do anything for the taste of the water which even the simplest filter fixes. I think you can get combo filter/purifiers, i.e., that kill pathogens by passing water through some kind of micro-nastyphobic membrane and also have a charcoal-like filter to improve taste.
+1 on the chemical additives; use only in emergencies.
All that being said, if I were traveling around the world, I'd want something that kills virii as well as amoebae.