For cooling water I would ...

For cooling water I would recommend Capacitive Deionisation (CapDI). It is an electrcally driven membrane process, that removes hardness without chemicals for regenertion like the use of HCl or NaOH normally used for regenration of IX softeners. It does not increase salinity. Reccommend to go to the Voltea website. Gives a good explanation and it increases the recycle ratio for cooling waters very well against acceptable cost. Good reference base.

For drinking water it is usually a matter of cost. For this salinity RO and ED are both in the same ball park. So you need other considerations like operability, recovery, scaling and sanitary or pathogen removal.