Buy iodine and use it when you need. I have a great msr filter that works awesome, but it weighs too much and takes up too much space. I drink about 70 % of my water with no treatment straight out of high country springs that flow from the granite faces here. But iodine is all I use for treatment now.
For Christmas I got a Sawyer Water Filter System and it only weighs 3 oz.! Its the lightest water filter system ever made that I know of! Cant beat that! Also for back up I bring Katadyn Micro pures which weigh one or two ounces.
A huge +1 for the Sawyer filter. We adapted the filter and spliced it directly into a Platypus line for the most streamlined approach. Hmmm, elk wallow water...:)