I'm new to this game, and would like to know when I should use a pick instead of spade for digging/destroying blocks.

When you need to take out a single block.
Also, the pick gives you the block you're destroying, to build stuff. Spade doesn't.

The spade removes the block above and the block below the block you're aiming at, making a space where you can run through without crouching. The spade doesn't supply you with the blocks you removed.

The pickaxe removes 1 block at a time and from what I've seen takes about 3 hits to remove, this supplies you with the block you removed unless you already have 100 of them.

Ok, thanks.

Spade is good for trenches and tunnels and destroying enemy buildings

Pick is used for everything else