Good question. Quite simply, choosing between either low-side switching or high-side switching can based on a few things: 1. Your design requirements for your system. 2. Your wiring hookup preferences or requirements. 3. The load you are driving. Acromag's high side switching products drive 250mA/ch whereas the low-side products switch more (ie; 500mA per channel). The price is the same for either, so it's basically a matter of choice, preference, design requirements, and load driving capability.