Have you thought about Demons/EA ? It's EXTREMELY tanky.
Anywho....between Demons and ninjas, I would lean just a bit more towards Demons. Ninjas are cool and all, and can pump out some damage....but they have like no resists. Having the ice shields on them would cap them, but when something gets thru it, MAN it hurts.
Demons would have a nice mix of Def and Resists...plus one of your pets will heal also. I feel like Demon Prince is a better pet overall compared to the Oni. I like throwing in the Impeded Swiftness proc in him since most all of his attacks has a chance to trigger it.
Demons= safer, good mix of damage types, good mix of Def and Resists, heal
Ninjas= can pump out major damage, hardly any resists, last upgrade will allow pets to heal. Better kill fast before the enemy gets to you