They are both absolutely identical.
If you are doing starting area at 1000xp/hour then it takes 1hr 30 mins to get 1500 xp. If you use the quest it takes you 1hr to get 1500 xp. So you save 30 mins getting those xp. If you are doing Jade Forest at 10m xp/hour the it takes 1hr 30 mins to get 15m xp. If you use the quest it takes 1hr. So you save 30 mins getting those xp.
If it takes 40 hours from 1 to 90, it takes 39.5 hours if you pop one quest as soon as you can. It also takes 39.5 hours if you pop one quest when you are getting max xp / hour.
Stop thinking of XP as a milestone it means nothing - only thing that matters is
TIME SAVED, and that is thirty minutes