Lever

The Mechanic stocks them, so none of this happens until you have found her in the Dungeon and given her a house. Once she is in, levers are cheap enough that you should never be rationing them.

Electrically they are identical. The difference is physical. A lever takes up a square of space instead of a single tile, which makes it far easier to spot across a room and far easier to click while something is chasing you. That size is also its restriction: it needs a flat surface underneath it the way furniture does, or a background wall to hang on, and it cannot be stuck to the side of a block. A Switch can, which is why cramped wiring still uses them.

The two positions look like on and off, and the wiring does not care about either of them. Flipping the handle sends a signal, full stop, and it does so in both directions. A dart trap on the far end fires when you flip it left and fires again when you flip it back. If your build needs to know its own state, that logic has to live in the circuit, not in which way the handle happens to be pointing.

Since the handle position carries no meaning to the game, you can make it carry meaning to you. Paint the two halves different colors and you can read the setting at a glance from across the room. That pays off most on things that are not really on and off, a minecart track junction being the usual example, where otherwise you are left guessing which way sends you where. Stick to non-deep paints and only the ball recolors, leaving the arm and base looking normal.

Platforms count as a valid surface, which is handy for wiring a walkway without cutting into it, and background walls take them too, so a lever can sit in the middle of an open room with nothing under it.