Toby,
little bit awry there mate.
In general terms, 'purification' is a two-stage process:
1. Filtration or clarification (e.g. with a Millbank bag, pump-action or gravity filter or by settling/floculation, such as a Gypsy Well).
2. Disinfection (e.g. chemical method such as iodine/chlorine or physical method such as UV or boiling).
Some filters are just that - filters. They do not 'purify' because there is no disinfection stage - you would do that by adding chemicals or boiling after you have filtered.
The very fine filters may use ceramic or a membrane, they can be susceptible to damage from freezing, they are more prone to becoming clogged and they can have low flow-rates.
A more porous filter will be less susceptible to damage, less prone to clogging and will have a higher flow-rate. Any viruses that get through will be zapped by your disinfection method.
I have used most of the available kit, including in humanitarian aid situations in the developing world.
I would recommend a Katadyn Hiker (a filter) or a Katadyn Vario (a filter).