Amanita is a 3HKO/Head secondary, it has the same weight as holding the Emperor's Katana, has higher ammo and it has VERY low recoil.
The Powerful Gift is a back-up sniper with a 2HKO/Head but weighs more, the recoil kicks high and the ammo count isn't all that great for a secondary.
Amanita is perfect if you're the type who likes stability, mobility and reliability or Powerful Gift if you're a sniper-focused player.