On my (now 17 after 2 Dungeons) 15 CNJ, i was getting 3k xp per Gold Rank at my level.
I get 500 xp per trash MOB kill in a dungeon.
Dungeon takes 30~ minutes or so to finish.
Dungeon provides me with loot to sell via MB or to wear and make the next run faster.

Skip to level 40 on my Monk while leveling that, at 40, FATE gave me 10k XP for Gold.
I get 950 xp per trash mob kill.
Dungeon takes 45~ Minutes to clear.
Dungeon provides me with loot to wear or sell, and Allegan currency.

FATEs are nice, but usually provide no better Experience per completion than a Sidequest. Dungeons are fine, FATEs are fine.