Cobalt Pickaxe

Both versions of this tier do the same job and the split is not about raw speed. The drill starts faster but ignores every mining speed bonus you own and reaches a shorter distance. The pickaxe accepts those bonuses, so once you are wearing mining gear or drinking the potion it pulls ahead, and the extra reach means less repositioning while you tunnel. With no speed bonuses at all, take the drill. If you build for digging, take this.

This tier gates the next one. Nothing you owned before it will touch Mythril or Orichalcum, so the moment cobalt appears the rest of the Hardmode ore chain unblocks. Palladium worlds get a slightly stronger version of the same tool, which matters far less than simply having one.