I'll second 300WM's recommendation. I used a Steripen adventurer with Katadyn Micropur chlorine dioxide as backup for all of last year; probably a total of 30 days in the field. I had no problems with either circuitry, battery failure or GI distress. The Steripen weighs 3.6 oz with batteries and the batteries will last through at least 40 liters of water by my count. Extra batteries weigh about an ounce. The water you treat has to be pretty clear or the UV is much less effective; I use a coffee filter if necessary and if the water is really murky (not too uncommon in Arizona) then I go ahead and treat with the Micropur as a follow-up. The thing I like about the Steripen is that there is no swimming-pool-water taste when you use it.
May I walk in beauty.