To an extent, this is an opinion question, and either option could be supported. I think defending the destination option is more difficult, so I think the better option is to say that the journey is more important to Santiago.
As the question states, the journey is what "ends up" being more important to Santiago. If readers can only use the first 80% of the book, then the destination is much more important to Santiago. His whole entire goal is to find the treasure and live out his personal legend; however, by the end of the story Santiago realizes that his final destination was exactly where everything began in the first place. That final destination would be meaningless, though, without the preceding journey. In order to be the man that Santiago is at the end of the story, he needs to go through all of the trials, successes, and failures that happen during his journey.