I leveled very quickly doing instances on the side, it was ususally 2 runs to get a level when you were the first level of availability then 3/4 to get the next. So say you have a level 22 Duty, you are level 22. After 2 runs (usually because you get the bonus for a new person) you should gain a level, at level 23 you will need 3-4 dungeon runs depending on speed of completion (bonus xp for a faster completion). In the end the gear you get does actually make a noticeable mark on your damage while leveling, the scaling drops off quickly but the next place does wonders.

The best part of dungeon grinding is that once you hit Stone Vigil you start receiving Tomestones which is the end all grind anyway. Nothing you can grind from fates or GC will touch the gear that Tomestones can lead to... so you may level a tad slower but you will be ahead of the curve come 50.