You do have to make some decision on where you are going to be getting water from, and how likely various problems are. There's no point in getting something that will see you OK taking water out of the Ganges in Calcutta if you are only going to use it in upland areas of the UK where the only problem is a moderate chance of giardia (a protozoan), and the effluent from the odd dead sheep.
The smaller the holes in the filter, the quicker it's going to clog up. Bear in mind also that activated charcoal for removal of nasty chemicals often lasts a very much shorter time than the filter it's attached to. "Smallest bacteria are 200nm" is only of any relevance if they cause illness in humans and are likely to be found in drinking water supply locations. Otherwise it's just marketing waffle.
I use the
Katadyn Mini, which is cleanable so it lasts a decent time, takes up not much room in my pannier, and which doesn't have much to go wrong with it so I don't find it's got dead batteries when I try to use it.
