Every situation is different. I do a lot more joining than mapping, but I do a lot of mapping too. If one way was always better than the other way, there'd be no reason to have both options in QlikView.
It cant be stated generalized. It all depends on what the situations needs.
But I'd classify whether the data to be replaced is huge or not. If not, I'd rather go for mapping, assuming it consumes relatively lesser resources (as join is a memory-heavy concept)
