As most trucks and many older cars come with leaf spring suspensions from the factory, the debate over which is best has a complicated history. Though coil springs have been around for nearly a hundred years, the leaf spring's history dates back well over a thousand, so both have long proven their respective merits. Which is better will always be a point of contention, but the answer largely boils down to the intended use.
A leaf spring suspension is made of a series of long, relatively thin sections of springy metal attached at both ends to a frame and suspending the axle in the middle. Coil springs look just like one imagines a spring would, and sits on top of the axle or lower control arm and the chassis.
In terms of function, leaf spring suspensions are much simpler, since the axle is suspended by the spring, and does not require the complicated suspension geometry of the coil-spring set-up. Leaf springs are also much sturdier, and are capable of handling much higher loads with less deflection than coils. Trucks with leaf springs are also easier to raise or lower.
Coil spring suspensions offer more range of suspension movement, and allow the user a wider tuning envelope through the suspension range than leaf springs. Practically all high performance applications use coil springs where possible and budget permits. Coil spring suspensions also usually perform better, having better engineered geometry than leafs.
The leaf springs' simplicity is as much a curse as a blessing. Since these spring attach at fixed points on the chassis, they give very little room for adjustability of suspension geometry. These springs also flex a great deal less than coil springs, resulting in a loss of wheel-to-ground contact under extreme conditions.
The two main drawbacks to a coil spring suspension are cost and load-bearing. Cost isn't so much an issue, if the vehicle was originally equipped with coil springs, but retro-fits can be very expensive and time consuming. Coils are not generally favored for very heavy vehicles, as the coil on axle setup isn't nearly as stable or strong as a proper leaf spring.
For heavy, industrial or budget-limited applications, leaf springs will usually do an acceptable job and prove themselves a durable choice. However, there are very few applications which will benefit from leafs over coils where performance is concerned.
