Undermount drawer slides and ball bearing drawer slides both serve the same purpose of enabling drawers to pull out and push in with a smooth, easy motion. Installation is simple and once installed, it will be difficult to tell which of the two slide types your drawer uses. However, tehre are a few differences to take into account when deciding which type of slide to get.
First, consider the type of slide you currently have. While undermount drawer slides can be installed on the bottoms of any types of drawers, ball bearing drawer slides install into grooves that stretch the length of each side of a drawer. To use ball bearing slides, your drawers need to have those grooves. Occasionally, drawers made for ball bearing slides will have slightly less area inside them because the sides need to be a little thicker to accommodate the slides themselves, as well as to support the weight of the entire drawer. Generally, counter units will be designed to use one specific type of drawer slide since ball bearing slides require less space in between the drawers, while undermount slides require around an inch of separation for the slides to fit.
While undermount and ball bearing drawer slides may be installed in different locations, they inherently serve the same purpose. They both function to allow ease and convenience in accessing the contents of your drawers. The vast majority of manufacturers install safety stops in their slides to prevent your drawers from pulling out too far, falling, and potentially breaking and harming the contents therein. Installation of both types of slides, regardless of where you install them, is relatively easy and pain free, though undermount slides require a bit more care. Since there are no grooves to mark where the slides go, you will need to measure the spacing between each slide, as well as ensuring that each slide runs straight up and down the bottom of the drawer. Maintenance on both types is as simple as occasionally applying some oil to the tracks to keep them sliding freely.