On a mechanical boost gauge a hose from the boost source has to run from the engine all the way to the gauge and the gauge has a diaphragm and mechanical mechanism to that moves the needle on the gauge. On an electronic gauge the pressure is measured by an electronic sensor and transmitted to the gauge electronically by a wire. Electronic gauges are usually more accurate and have more options, like memory or peak hold and can be logged by dsmlink and are easier to install but they can be more expensive.