mono leafs are more desirable for drag racing due its reduced weight. Multi-leafs were for increased weight capacity and stiffens the ride and reduces wheel hop and sag under acceleration. Both work fine in a street application, but with monos you need to run a traction aid as the spring tends to wrap up more vs a multi leaf. Multi leafs can be set up like the old chrysler super stock springs so you don't need to run a traction bar. You change the spring rate by moving the leafs closer to the front of the car and clamping them down together, makes them stiffer in the front half so they wont wrap up. One draw back is the car can sit up higher in the rear.