MA tends to be a little endurance hungry so Willpower or Regen would be good choices IMO; both have Quick Recovery which, if taken along with Stamina; will negate the end drain. Willpower a little less so given the toggles that you need to run with it. Regen only has one toggle in the set.
Regen as you may know is a hitpoint, regen/self-heal based defense and willpower is a good mix of resistance and defense. Both secondaries are very different but good at what they do.