Spider curls hit the short head of the biceps. The big advantage of spider curls over the preacher curls is the constant tension, especially at the top portion of the movement. The slow negatives allow more time under tension during the eccentric or lowering phase, where we can handle more load.

Here's an additional twist: use a fat grip. One of the benefits of thick bar training is that thicker handles stimulate much more muscle activation in the hands, forearms, upper arms, and even the whole upper body. More muscle activation means much bigger muscle and strength gains.