Mine Blocks is an earthbending ability that breaks blocks directly in the direction that the earthbender is facing (creating something that looks like stairs). Its a utility ability for exploring caves and going underground, easier and faster. It is also good for hiding against your opponents and tricking them.
As with all other abilities, there are two choices for Mine Blocks' level IV upgrade.
Long Tunnel: Can mine very far, with level III fortune
Long Tunnel allows you to create really long tunnels for fast underground exploration. It also gives the ability a fortune III enchantment which is great for gathering extra resources in your way down or while mining ores. It's a balanced path because it mines really long distances and has fortune III.
Vein Miner: Mines out ore veins with level IV fortune
Vein Miner let's you mine out entire veins of every ore in it's path. It adds fortune IV enchantment too which is extremely beneficial when it comes to mining resources. Compared to Long Tunnel, this upgrade is recommended to earthbenders who want to benefit from the level IV fortune aspect.