Aether or Bonfire is fine. I gave mine both, but have been leaving Aether equipped mostly. Ike will trigger Bonfire twice when attacked thanks to QR - the timing works better than Ignis.
Aether is probably what you want for chain challenges to keep him fueled. I've found it to be pretty useful in arena assault too. If I see an Armor team, QR Aether Ike needs little assistance to solo them.
---
http://myanimelist.net/profile/AntiquatedNotion