Depends on need, usage, and other details.
For hiking? Neither. Most water in the backcountry is pretty clean, the only threat are easily filtered parasites and common bacteria. A simple, lightweight, cheap filter is the only way to go. The Sawyer springs to mind. 3oz, $50, and a million gallon capacity.
For more urban usage, or for your basic "world gone mad" kinda stuff where you have no idea what's in the water, whether it's chemicals, virulent viruses, or alien DNA, you really need a purifier. I like the First Need for portable purification. It's capacity is rather low, so plan on a couple extra filter elements. But it's the most effective purifier I've seen that is even remotely portable. It weighs a lb, which isn't too far off from the Katadyn and MSR products, but filters far more, and for about the same price.