PvE:
Hard to compare.
Mystic can kill stronger boss. Or get away if his summon dies 1hit.
Seeker can kill much bigger group of mobs at once.
Mystic can partly replace Veno or Cleric.
Seeker can partly replace BM or Barb.
PvP:
Good Mystic can attack harder.
Good Seeker can survive longer.
Different play style, different purposes.
I honestly don't know.