There is no difference in cache loot if the cache was obtained either in normal, hard, expert, master or even torment.... Only difference is getting torment specific legs from elites you may encounter during your bounties...

If your aim is to farm cache only and farm as much as possible in the shortest of time, normal difficulty is the best option...  There is no point farming Torment or even hard or above...

Unless your aim is to have fun while getting the cache as an extra bonus ... Then yes... You may choose whatever difficulty your heart desires...

Horadric Caches
Now have an increased chance to drop Legendary items on Torment II - VI

go to normal mode clear all bounties in act 1 then claim the cache , keep around 14 cache then open up all in leah bedroom , if u get is profit if not rinse and repeat u will still get it , i got another one the other day just farm 3 cache in T1 already get it and the stat is quite marvelous with 50 CHD