For me I always go maxed on treasure hunter ability, my reasoning is:
a) Give you more exp (Admittedly still low, but a lot higher then lvl 1, and when you use it every battle it adds up).
b) cost only 8 rage.
c) Allow you to dig 3 treasures per battle. I use this to the max every battle, this amplifies point a).
It is true that at a certain point each chest cost you almost 3x rage cost, but once you get the -rage cost lvlup it will only cost 8 rage, at more then double the xp for dragon and 3 chests per battle. I'm a fan
