We are asked this question every day and it is hard to provide a one-size-fits-all answer.  Here are the issues:

Axle wrap (unwanted deflection or bending of a spring caused by torque to the tires) is more often a problem with spring-over setups than with spring-unders. Axle-wrap can be major or minor to fix, depending on numerous variables such as lift-height, spring flexibility desired, vehicle weight and load, engine power, and gearing.  We have made changes in our spring design to help stop the bending of the springs without hindering flexibility or compromising the ride quality.  But if you have a torquey V8 and low gears, you may have more of an axle-wrap problem than the springs themselves can handle, and you may need to install a torque rod of some type.

A spring-over setup is going to articulate easier than a spring-under mostly because the spring-over is flatter so the main leaf can roll up on its side (twist) easier. The spring-under setup in a 5″ lift or higher will not articulate as easily in a light-weight vehicle. But, for example, a spring-under Wrangler with 300 pounds of extra weight in the rear and a bumper and winch on the front could do as well as a spring-over (just with less ground clearance). 

We are also finding problems with spring-overs on stock weight or lighter vehicles that are used for rock crawling. When the vehicle is put in a very steep uphill climb, it puts all the weight of the vehicle on the rear axle. In most cases, this will bend the front half of the rear springs.  We have made some design changes to help with this problem.

Orbit-Eye bushings are a great way to improve articulation in either spring-over or spring-under setups because the main leaf does not have to roll up on its side (twist) as much as with stock bushings. The lengthwise twisting action of the spring when articulating shortens the life of a spring, so the Orbit-Eye bushings help with that problem, too.

We build as many spring-overs as we do spring-unders and we find that the issue is not which setup is right or wrong or better or best; it is what best suits your needs, the specific rig you are outfitting and how it will be used.