I find that Zero tablets aren’t as effective as Nuun tablets. According to the recommendations of Zero tablets you should use 2 tablets for a 1.5l bladder. This will have 500mg of sodium.
Zero tablets have 250mg sodium per tablet.
Nuun have 700mg of sodium per tablet (Nuun recommend 1x tablet per 500ml).
So to fill a 1.5l bladder you need 2x Zero tablets or 1x Nuun tablet (if stick with the High 5’s recommendation of sodium intake required).
Therefore I reckon Nuun tablets are better value.
EDIT: Information above is wrong, please ignore. Though I still prefer Nuun.