Story quest or map clearing?
Story quests mostly give quite a decent chunk of XP, I’d suggest trying them and see how it goes
DODGE!!! – https://www.youtube.com/watch?v=kvS6zMThiZU
Depends on how quickly you think you can ‘catch up’, fastest would be doing both. Getting up to the lv60 personal story would also get you 2 Black Lion Keys and 400 Influence for your guild which is worth 80s.
Story steps reward exp based on the step’s level and map completion rewards based on the map’s highest level. You are getting less and less in terms of percentage of a level the further you are away.
It depends what it is you want to do faster.
It would be faster to level up by doing your story quests first because they give big chunks of XP, and as Khaisanth said they’re more beneficial when you’re closer to the level required by the quest.
But you can complete the story faster if you wait until you’ve completed the map and then use waypoints to get to the start of each instance instead of having to run there. (Although this will also be more expensive because waypoint costs go up as your level does.)
Or you’ll complete content faster by doing map completion and the story together – let the story guide you around the maps and complete them as you go. That way you won’t have to double back but you out-level the story and won’t get as much of a benefit from the XP.
“Life’s a journey, not a destination.”