In order to unlock Dragoon you have to meet the following requirements.
Marauder at lvl15 and Lancer at lvl30.

So whether you choose to level Marauder to 15 first or you choose to level Lancer to 30 then Marauder to 15 is really up to you. Regardless of what you do you will still end up going back to Lancer in order to do the Dragoon quest and become a Dragoon.

Hope that helps!