Objective: How to purify your water manually?

Which of the following solutions is more sound in terms of naive physics reasoning?

Select from:
(a). For water purification, carry a Katadin water purifier without spare filter element and Potable Aqua water purification tablets. They can purify up to 500 liters of water each,;
(b). For water purification, carry a Katadin water purifier with spare filter element and Potable Aqua water purification tablets. They can purify up to 500 liters of water each,;
(b).