At level (Rohan Gold) 50, you can move on to sub-classes. 2 for each class. For dekan- it is drag knight and sage. Dragon Knight Good damage, but lacks defense abilities. Focused more on PvP imo. Dragon Sage Pretty good damage, little attack skills, but has tremendous buffs such as the defense buff.
Basically, dragon knight seems like a better class (Rohan Crone) because of its skills. But in the long run, the dragon saint would benefit more, because when you reach higher levels of magic and sage fan and knight. Again, this have not been tested, this is pure justification that we have gathered from asking other people and reading forum threads about dekans.
In short, if you want to go to PvP and then, because it has the DK better PvP skills. If you want to go immediately to give the DS because they have good skills, such as defense force increases. Again, not all of this information is the best. No guarantees. These are just guidelines that we have collected for Rohan players.