Aggramar's will be higher dps for almost every tank spec in M+ making it the choice. Aman'thuls is trash for tank dps when you can't activate the secondary proc and no tank is going to rely on a random leech proc for survival.
Amans would of been great like 8 weeks ago. It's meh for tanks now. Alright in raid, but not much of an upgrade and a downgrade in M+.