exp might be still a bit lower per hour in dungeons, but you get also awesome loot too.

so if you doing just fates until lets say lv40, you miss great gear to survive lv40-50 dungeoins i would say.
so a good mix tho fates and dungeons is the way to go.
if you want there are still many quests for 2-3 classes to get you to 50 aswell wich provide decent gear