Chlorine dioxide.

Here's some reasons why:

Once opened, Iodine tablets expire after a year. 4 years if left unopened. Chlorine dioxide tablets are individually sealed and each tablet is good for 4 years if left as such.

Chlorine dioxide is effective against cryptosporidium, iodine is not. Both have a treatment time of 30 minutes for water that is clear and of roughly room temperature. The exception being water that is murky or very cold, in which a 4 hour treatment time is recommended for chlorine dioxide against cryptosporidium.

Finally, ingestion of too much iodine over time can lead to thyroid problems and iodine does not evaporate from treated water. Whereas, chlorine dioxide will eventually dissipate from the treated water if the container is left open and given ample time.

The big downside for chlorine dioxide is cost. They're more expensive than iodine tablets. However, if I was truly cost conscious I would be treating my water with liquid bleach anyway.....which is effective against all the same waterborne illnesses and parasites as iodine tablets, but is a lot cheaper. Like iodine though, liquid bleach is ineffective against cryptosporidium.