I m lv 35, already unlocked Arthur. Which legendary soul should I unlock next?

A few additional questions...
1. Is Sigfried usually the best choice for most rage metered bosses? (Raid / Advent)
2. In what situations will Mortred be better?
3. Are the defence / healing souls worth getting?

It seems to me right now that burst damage is the way to go. And Arthur / Sigfried looks really good in the dmg department.

P.S. Solomon of course is best used for sucking dick.