The story lines (class quests) vary just a wee bit depending on your choices even in the same class, all classes, though similar all fit together like puzzle pieces to create an entire story line. Most of the side (planet quests) are the same or very similar among all classes.
There's just enough of a difference and similarity that I don't get too bored or too lost with my 8 characters.
Not sure how your guild works, ask an officer
After my 2nd character I no longer did all the quests, give space missions a shot around 20 and see if the guild does Flashpoints. Then you can just do the Class and planet story quests.