One has to keep in mind that there are Filters and there are Purifiers.

Your typical camping/hiking filter removes protozoa, parasites, and most bacteria. What it really fails to filter out are the majority of viruses. Viruses like Hepatitis A, SARS, Polio...not nice things.

For that, you need a purification method. While there are combination filters and purifiers, most people rely on after-treatment.

Chlorine dioxide tablets or drops is a very popular after-treatment method. However, boiling will also kill viruses (at sea level 1 minute of boiling time is recommended, at 6500ft+ elevation 3 minutes is recommended).

Essentially, filtration and purification go hand in hand.

However, if I only had room for one, I would make sure I had a method of water purification (like Chlorine Dioxide/Micropur MP-1 Tablets). Why? It will still make water completely safe to drink, you might just have to wait longer for it to be effective. Whereas, your typical filter is bulkier and can still fail to filter dangerous viruses.