Quick take
Use carbon when chlorine taste is the main issue. Use RO plus remineralization when source water is very hard or has broader dissolved-contaminant concerns.
Taste problem: chlorine
Activated carbon is often the simplest fix for chlorine taste and odor.
Maintenance problem: scale
Hard water leaves mineral deposits on heating elements and in narrow internal passages.
RO for difficult source water
RO can reduce hardness and many dissolved contaminants, but ultra-low-mineral water may taste flat unless remineralized.