More info here:
There are differing opinions, and yes, LR and bioballs both do the same thing for converting ammonia to nitrate.
The difference is that LR has anoxic zones where different bacteria grow, and convert nitrate to nitrogen gas. Unfortunately, these bacteria are limited by their carbon source, so not all nitrate is converted to nitrogen and nitrates still build up in our water.
Bioballs, and other bio media, were designed to optimize aerobic conditions for denitrification in sewage and wastewater systems, where they work quite well. However, in a reef tank, they do not have the anoxic zones that LR has, and so ALL of the nitrates produced enter the water column, and none of it is converted to nitrogen.
By adding bioballs to the tank, it removes some of the aerobic denitrification from the LR and now puts it in the bioballs. By doing so, the nitrates produced in the biomedia must now travel through the water column to the LR so that it can be converted to nitrogen. This can result in a small increase in nitrate concentrations in the water column.
My personal opinion is that so long as the biomedia does not house a large portion of the denitrifying bacteria, or if you are not dosing a carbon source (advanced only), then it is questionable whether you would actually see a measurable difference.
However, why take the risk? The LR does the job, and the biomedia is unnecessary and can trap detritus. If you want to use rings, balls, or even LR rubble, it is recommended to put it in a filter sock for easy removal and rinse it with every water change to remove any accumulations.